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?
Perhaps I just haven't thought about this enough and don't foresee all possible problems. My suggestion applies only to simple models. Composite models and the WCS should be well defined. 
But for a simple model which is initialized by some parameters, isn't it sufficient to know its name and named parameters to create it? How will a polynomial be represented? The number of parameters is going to depend on the `degree` parameter. If its schema accepts variable parameters why can't any model schema accept any number of parameters which are passed to the model? 
It's possible I don't understand all problems that this may lead to.