Consider Switching Inheritance Order of `CoilSet` and `MixedCoilSet`

[dpanici]

This is somewhat unrelated, but would it make more sense to have MixedCoilSet be the parent class and CoilSet as the child? Since MixedCoilSet is the most general and CoilSet is the special case where all of the coils have to be the same type.

Originally posted by @ddudt in https://github.com/PlasmaControl/DESC/pull/1106#pullrequestreview-2162097544