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?
I think where there are variable number of parameters, we should use a list, i.e. for a list of polynomial coefficients.

I think many of the polynomial transforms will end up having the same schema (i.e. having a list of coefficients).  But many the projections will have different kinds of parameters -- usually a fixed number.  An affine transformation would take a matrix.  So the nature/names/numbers of parameters will be different between different transforms, even if certain classes of them will be identical.