Create new Chiron topology

[chrisiacovella]

Currently, chiron.topology uses the openmm topology object. This was done initially for convenience (being able to quickly import openmmtools test systems during initial development/testing).

Given that we will not be using most of the features of the openmm, and will need many specialized functions to infer topology, we should create a rather minimal topology object. We should probably include functions that accept openmm topology objects as well as openff-toolkit objects.

We should be able to easily using openff-units for defining element information (e.g., masses, atomic_number, symbol, etc.).