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 -- this is all of the transformation types that could appear at any location in the transformation "tree".

There unfortunately isn't a way to handle infix binary operators in YAML without parsing strings.  I'm *really* trying to avoid parsers on top of parsers here.  I think it's important to be able to say "if you have a YAML parser, you can parse FINF".  In the present design, binary operations end up looking/feeling like LISP, with prefix notation, i.e.:

```
summed:
    transform1: