[WIP] Simple MM-GBSA implementation

[jchodera]

Description

This PR provides a simple MM-PBSA-like implementation that computes the difference in enthalpy between complex and protein+ligand using OpenMM for PME, RF, or GBSA simulations.

Todos

Notable points that this PR has either accomplished or will accomplish.

• autodetect explicit or implicit solvent
• create command-line driver with entry point
• allow user to specify PME, RF, or GBSA simulations via CLI
• allow user to specify convergence tolerance and maximum number of iterations or wall-clock time
• make sure to clean up contexts
• add unit tests and example input