Refactor wcs_ref_models.py to reduce repeated methods

[emolter]

All of the models defined in wcs_ref_models need identical on_save and to_fits as well as much of the same functionality in their respective validate() methods. There is already a base class named _SimpleModel that contains these methods. However, there are 11 different models that do not inherit from _SimpleModel, so lots of repeated code remains.

It would be useful to refactor this (using, e.g., a mixin pattern) so that all the models can benefit from the implementations in the base class or classes.