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.
In the 1D case they are just `c0`, ..., in ascending order but in the 2D case the order is
1st dimension coefficients in ascending order, 2nd dimenson in ascending order, mixed coefficients. If it's a dictionary I don;t have to know the order. Well, but I have to know the names. I thought the latter is easier but I agree that in the case of a low degree polynomial the former is easier. That's why I have two options for the coefficients there (ndarray or a dictionary). I don't mind changing the ndarray spec to array which will turn into something like

`coeffs: [[1, 2], [3, 4]]` for multi-dimensional 1D polynomial, meaning `c0=[1,2]` and `c1=[3,4]`

Is this better?