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.
My understanding was these schemas are tied to an implementation so that models can be initialized.
If this is not the case, this model should be written in terms of coefficients only (the degree should be dropped) and the rest is up to pyasdf to figure out.
 @mdboom Is this correct?
I think writing out multidimensional polynomials is still desirable. The same is true for `Shift` and `Scale`. It's optional so it's not limited to astropy.