Adds a custom function to EPrints' repository view configuration, so you can define a
subroutine for each menu level that returns the map of {id=>count}
for that level.
Particularly handy when the a conflict between the menu's fields and the generated
DISTINCT BY
clause include NULL values in the menu despite allow_null=>0
being
specified in config.
See the discussion at http://mailman.ecs.soton.ac.uk/pipermail/eprints-tech/2017-September/006807.html