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.
Ok -- I know the three of you have spent a lot more time on the Polynomial model in astropy.models than I have, so I leave it mostly to you to work it out.  To sum, it needs to be general enough and efficient enough to store the large polynomials that may be required.

I don't think there's much benefit to supporting *either* an array or ndarray.  If the data is ever more than 1-dimensional (which it sounds like it can be), just use ndarray.  That doesn't require that the data is in a binary block -- it could be inline.