Create a method to compose flavor transformations

[sybenzvi]

Do we want snewpy to be capable of stacking a set of flavor transformations? For example, given a model we apply some resonant effect in the star, followed by vacuum oscillations, followed by MSW Earth-crossing oscillations.

Currently the interface to implement transformations is get_transformed_spectra(self, t, E, flavor_xform), so we could take the existing flavor transformation classes and setup a way to compose several different processes into one object that gets passed to get_transformed_spectra.