Investigate whether masked tables can be used as reference catalogs. Insure consistency across all top-level functions. Document

Investigate whether masked tables can be used as reference catalogs. For example, it appears that `fit_wcs` would use masks but `align_wcs` will ignore them. While not technically a bug as support for masked tables is not stated (and masked tables were not intended to be supported), inconsistency is not desirable. Document support or non-support for masked tables.