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.
I was wondering about this too. The 2D polynomials I'm working with now have 21 coefficients. These will be stored in a 2D array. If `param_dim != 1` the coefficients of a 1D polynomial will be stored in 2D arrays too. This was the reason for defining this as an ndarray but I don't mind putting them in an array if you think it's better.