[WIP] FrozenSetDict; FrozenSetCounter

[dwhswenson]

To handle contact pairs, we frequently use dictionaries/counters where the keys are a frozenset of the indices. This gets very annoying to deal with, especially when writing unit tests (so many things need to be wrapped in frozenset).

This PR creates a couple simple classes, FrozenSetDict and FrozenSetCounter, that use the frozensets internally, but allow the user to access them with tuples or lists (of either indices or the actual objects).

• FrozenSetDict
• test for FrozenSetDict
• FrozenSetCounter
• tests for FrozenSetCounter
• use FrozenSetDict/FrozenSetCounter in main code wherever useful
• update old tests for readability (use lists instead of frozenset wrappers)
• docstrings