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?
From what I see Nadia saying, one does need to define the parameters for each model. Is the issue over variable numbers of parameters or something else? My inclination is that we start with a rigid scheme and then open up if needed.