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?
What I am saying, the details aside, is that I am in favor of a more loose schema, where it requires a model name, its (names) parameters, param_dim and possibly inmap and outmap (I'll explain the last two later).