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 this case is there a neeed for different schemas for 1D and 2D polynomials. Something like this will work in both cases:
```
type: object
properties:
  coefficients:
    description: |
      An array with coefficients.
    type: 
      $ref: ndarray
```
If there're separate schemas then coeffs can be of type `array` in the 1D case and type `ndarray` in the 2D case.