xarray CRS coord accessor

@xr.register_dataset_accessor('crs')
@xr.register_dataarray_accessor('crs')
def make_crs(xarray_obj):
    return xarray_obj.coords['crs'].item()


In [24]: ds = xr.Dataset({'x': 1}, coords={'foo': 'bar', 'crs': 'hello, my lovely unwrapped object'})

In [25]: ds
Out[25]:
<xarray.Dataset>
Dimensions:  ()
Coordinates:
    foo      <U3 'bar'
    crs      <U33 'hello, my lovely unwrapped object'
Data variables:
    x        int64 1

In [26]: ds.crs
Out[26]: 'hello, my lovely unwrapped object'

In [27]: ds.foo.crs
Out[27]: 'hello, my lovely unwrapped object'

In [28]: (ds + 10).crs
Out[28]: 'hello, my lovely unwrapped object'