Sortable pub date logic

**Default 'usable date' range is 500 to current year + 6**

MarcToArgot
  MarcToArgot::Macros::Shared::PublicationYear
    usable_date? - determine whether a given date value is usable for deriving a reasonable single value for sorting/filtering
      1997 is usable - 4 digits, in usable range
      688 is usable - fewer than 4 digits, but in usable range
      9999 is usable - gets translated into current year + 1
      499 is NOT usable - out of usable range
      6754 is NOT usable - 4 digits, but out of usable range
      1uuu is NOT usable - indicates only the millennium
      uuuu is NOT usable - unknown date
    is_range? - determine whether a single date value from fixed field should be interpreted as a range
      1997 is not a range: 4 digit date
      199u is a range: decade
      19uu is a range: century
      uuuu is not a range: unknown
      198| is not a range: 3 digit date
      66u| is a range: 3 digit date, decade known
    is_range? - determine whether a single date value from variable field should be interpreted as a range
      1997 is not a range: 4 digit date
      199- is a decade range
      19-- is a century range
      1--- is a millennium range
      198? is not a range: it is a best guess at the specific date
    get_date - translate a fixed field date into a single value for further processing if it is usable, and discard it completely from further processing if it is unusable
      19uu returns 1949 -- midpoint of range 1900-1999
      688 returns 688 -- no translation necessary
      9876 returns nil -- not a usable date for further processing
    get_date  - translate a variable field date into a single value for further processing if it is usable, and discard it completely from further processing if it is unusable
      WHEN 008/06 is not c, d, or u
        197- returns 1974 -- midpoint of range 1970-1979
        688 returns 688 -- no translation necessary
        9876 returns nil -- not a useful date for further processing
        c1999 returns 1999 -- extract only digits, followed by -
        -2018 returns 2018 -- extracts only digits, followed by -
        192-- (extra hyphen) returns 1924 -- extra hyphen ignored when figuring midpoint of range
        [197-?] returns 1974 -- ignores brackets and terminal question mark
        [2017];©2018 returns 2017 -- takes only the first date
        [between 1950 and 1959?] returns 1950 -- takes only the first date
        '-December 16, 2014.' returns 2014 -- takes only the year
        [n.d.] returns nil -- no recognized year pattern found
      WHEN 008/06 is c, d, or u
        192-- (extra hyphen indicating open date range) returns 1924 -- extra hyphen ignored when figuring midpoint of range
        [2017];©2018 returns 2018 -- takes only the final year date
    midpoint_or_usable - called when it looks like there is a range. Given the two dates derived using the get_date logic, tries to return the logical midpoint or otherwise usable value from the range
      1850, 9999: returns 1850, use date1 if date2=9999
      1850, 1900: returns 1875, the midpoint between dates since date1 < date2
      2000, 1984: returns 2000, since date1 > date2 (which is not a reasonable range!)
      1894, 1894: returns 1894, sincd date1 = date2 (which is not a range)
      1894, nil: returns 1894, the only usable date given
      nil, 2019: returns 2019, the only usable date given
    LOGIC BASED ON DATE TYPE AND OTHER FACTORS (\ indicates a blank character)
      WHEN DateType = b (No dates given; B.C. date involved)
        AND no 260 or 264 having dates
          (MTA) pub date not set
        AND record has 260 with |cp1999
          (MTA) pub date = 1999
        AND record has 264\0 with |c1999, followed by 264\1 with |c2000
          (MTA) pub date = 2000 (uses date from preferred 260 or 264)
      WHEN DateType = c (Continuing resource currently published)
        AND date2=9999
          (MTA) pub date = current year+1 (use translated date2 if it is a usable date)
        AND date2=2018
          (MTA) pub date = 2018 (use translated date2 if it is a usable date)
        AND date2=2uuu
          AND no 260/264 date present
            (MTA) pub date = nil (millennium range not usable, will not set pub date from date1 since that would poorly represent the currency of a currently published continuing resource)
          AND 260/264 date = |c1900-200-
            (MTA) pub date = nil (millennium range not usable, will not set pub date from date1 since that would poorly represent the currency of a currently published continuing resource; gets last date from 26X and treats as range)
      WHEN DateType = d (Continuing resource ceased publication)
        AND dates = \\\\1949
          (MTA) pub date = 1949 (date2 if present and usable, to represent the most current material that was published, since there is no start date)
        AND dates = 194519uu
          (MTA) pub date = 1972 (assuming pub cannot cease before beginning, midpoint of date2 treated as 1945-1999)
        AND dates = 1945\\\\
          AND no 260/264 date present
            (MTA) pub date = nil (will not set pub date from date1 because it would poorly represent the ultimate currency of a later-ceased continuing resource)
          AND 260 with |c1945-1984. present
            (MTA) pub date = 1984 (will not set pub date from date1 because it would poorly represent the ultimate currency of a later-ceased continuing resource; takes last year recorded in 26X)
        AND dates = 19459999
          (MTA) pub year = current year (assume record marked ceased without adding ceased date in error, and that such an error would be caught somewhat soon)
      WHEN DateType = e (Detailed date)
        AND dates = 20120215
          (MTA) pub date = 2012 (usable date1)
        AND dates = 02120215 (date1 typo for 2012)
          AND 260 with |c2/15/2012 present
            (MTA) pub date = 2012 (date1 unusable, cannot set year from date2, take year from first 260/264
          AND no 260/264 date present
            (MTA) pub date = nil (date1 unusable, cannot set year from date2)
      WHEN DateType = i (Inclusive dates of collection)
        AND dates = 19101950
          (MTA) pub date = 1910 (use date1)
        AND dates = 1uuu1950
          AND no 260/264 date present
            (MTA) pub date = nil (do not set date from date2)
          AND 260/264 has |c1910 to 1950
            (MTA) pub date = 1910 (do not set date from date2; get first date from 26X field)
      WHEN DateType = k (Range of years of bulk of collection)
        (MTA) works the same way as DateType = i (set only from date1 or first 26X date)
      WHEN DateType = m (Multiple dates)
        AND dates = 19662000
          (MTA) pub date = 2000 (prefer date2 if it is a usable, non-9999 date
        AND dates = 19669999
          (MTA) pub date = 1966 (use date1 if date2 = 9999)
        AND dates = 198u9999
          (MTA) pub date = 1984 (use date1, treating as range if necessary, if date2 is unusable
        AND dates = \\\\9999
          AND no 260/4 date present
            (MTA) pub date = nil (no usable dates -- 9999 not considered usable for this date type)
          AND 260/264 has |c1910, 1950
            (MTA) pub date = 1950 (get final date from best 26X field)
      WHEN DateType = n (Dates unknown)
        AND dates = 18501900
          (MTA) pub date = 1875 (use midpoint between the two usable dates)
        AND dates = n19661966
          (MTA) pub date = 1966 (use date1 if date2 = date1)
        AND dates = n20001950
          (MTA) pub date = 2000 (use date1 if date2 > date1)
        AND dates = n19842uuu
          (MTA) pub date = 1984 (use date1 if date2 unusable)
        AND dates = n1uuu2018
          (MTA) pub date = 2018 (use date2 if date1 unusable)
        AND dates = n1uuu2uuu
          AND no 260/4 dates present
            (MTA) pub date = nil (no usable dates)
            AND 260/264 has |c1910, 1950
              (MTA) pub date = 1910 (get initial date from best 26X field)
      WHEN DateType = p (Date of distribution/release/issue and production/recording session when different)
        (MTA) works the same as DateType = m (preferences date2)
      WHEN DateType = q (Questionable date)
        (MTA) works the same as DateType = n
      WHEN DateType = r (Reprint/reissue date and original date)
        (MTA) works the same as DateType = m (preferences date2)
      WHEN DateType = s (Single known date/probable date)
        (MTA) works the same way as DateType = i (set only from date1 or first 26X date)
      WHEN DateType = t (Publication date and copyright date)
        (MTA) works the same way as DateType = i (set only from date1 or first 26X date)
      WHEN DateType = u (Continuing resource status unknown)
        AND dates = 20129999
          (MTA) pub date = current year + 1 (9999 used in its translated form)
        AND dates = 19002uuu
          (MTA) pub date = 1900 (use date1 if no usable date2)
        AND dates = 1uuu2uuu
          AND 260/4 has |c1989 through 2017.
            (MTA) pub date = 2017 (use final 260/4 date if no useable 008 date)