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?
Any model that we want to interchange with another system would need to have a schema, yes, so it's clear what the representation is and how to use it etc.  However, you're right to point out that we need an escape hatch so that "user defined" models could also be used, even though a compliant FINF parser wouldn't be required to understand it.