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.
Though that does raise the question of what a good representation of that would be, and if it should be possible in this framework (the fact that astropy.modeling can represent such a thing efficiently is nice, but beside the point--it shouldn't be a requirement).