First draft of generalized WCS standard.

WIP.

At this point, I'm deliberately trying to avoid some of the rigidity of FITS WCS.  All FITS WCS have a `CDELT` offset and `PC` matrix, for example, as the "core" concept.  But I think that's limiting.  What if you want to correct on the focal plane first, for example?
For JWST I've been using JSON reference files and the schema basically describes a models by name and
parameters. I think something like this

https://aeon.stsci.edu/ssb/svn/jwst/trunk/jwst_pipeline/assign_wcs/jwst_pipeline/assign_wcs/reference_files/spec_wcs.json

should work.