Generalized WCS

Related to spacetelescope/finf-standard#14.
Indeed. And in the context of astropy/astropy#2580 the thing to do with these would probably be to not implement a `__call__`, but instead override `prepare_inputs` and `prepare_outputs` and provide a no-op `evaluate`.  Or they might be a special subclass of Model that is specifically for remapping inputs/outputs.  For now this is fine, but that will be the idea anyway.