Generalize finding of sigma layer variables

[veenstrajelmer]

Generalizing update needed in reconstruct_zw_zcc() for #648 and probably others:

# elif f'{gridname}_layer_sigma' in uds.variables: #TODO: var with standard_name='ocean_sigma_coordinate' available?
elif len(uds.filter_by_attrs(standard_name='ocean_sigma_coordinate')) != 0:

This keeps the unit/systemtests green, so seems like no-regret although good to test with other old sigma models (not included in postprocess_mapnc_ugrid.py acceptance test.