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.
Yes, `len(coeffs)` can be computed from `degree`. All polynomials are defined in terms of `degree` so that a model can be initialized without setting the coefficients so `degree` is required. However, if the ASDF file is used to store polynomial models, the coefficients are already known and can be passed to the model as named arguments. That's why I added an optional `coeff`.

`param_dim` is an integer specifying the number of models of that type in the instance. 
This creates 2 polynomial models of degree 1:

`models.Polynomial1D(degree=1, c0=[1, 2], c1=[1, 1])`

There was a lot of discussion about this on the astropy list but I think this is still valid in astropy master. @embray would know if this is supposed to change in the future.