offload all flavor transformations to snewpy

[sybenzvi]

We should remove obsolete neutrino flavor transformation classes in ASTERIA and let snewpy handle all transformations. Resulting changes include:

• Removal of the matter.py module.
• Removal of the oscillations.py module.
• Closeout of tickets Refactor SimpleMixing to Mixing #54, Error in simplemixing_class #64, and Error in supernova_mixing #66 as "wontfix."