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?
Is this where all transformation models would be listed? For example, a ND polynomial model?

I think we have been leaning away from the summed and more toward binary operators rather than the previous model for summed (or at least supporting standard binary operators). How would you add these? And then there is the issue of allowing arbitrary wiring of inputs to outputs of multidimensional transforms (e.g. use this transform for the second coordinate, and other more complex connections