Compare OpenMM and HOOMD for performance and ease of use

[ctk3b]

Until HOOMD becomes a properly importable package, it might be easier to play with OpenMM because we could call it directly within Python. It also happens to be easier to install for now (until HOOMD gets a conda package). My main question is does the run command release the GIL or are there other ways to launch multiple simulations without having to go out to a file. If it is indeed simple, then performance is the next question.

The basics are outlined here and tabulated functions are added via CustomNonbondedForces.

[ctk3b]

See #26