Add schemas for some astropy.modeling models

As I am reading through this and trying out pyasdf I tried creating schemas for `astropy.modeling` models commonly used for WCS transformations. So here they are.

I think only models which don't define an analytical inverse need the `inverse` definition in the schema so only the polynomials have it here. If this is acceptable I'll add the rest of the polynomials.
Well, as far as modeling goes, a good example is the nonlinearity case. We want to use the polynomial model for every pixel. Certainly in this case using ndarrays makes sense (how else to represent that case when you have millions of instances of a polynomial model).