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?
Yes, that's the problem with a standard, you have to be explicit about what has to be supported. If someone adds a new model, presumably they have to provide a schema of their own. Something free form could be done, but then we wander into the FITS zone, where everyone has their own standard (Which doesn't even have a schema). I'm not sure we should encourage that kind of use (well, I'm pretty sure we shouldn't).

At one point I suggested that we support a general expression syntax, but I think Mike convinced me not to. If we support most binary operators (as constructs, not as expression syntax), and many standard functions, the fact that we can combine all these does give users a lot of flexibility to define new models.