Cache openmm_system upon creation in OpenMMHarness

[dotsdl]

During #151, caching of the openmm_system was proposed as an optimization that could be valuable when we compute many gradients/energies for the same molecule in the same set of jobs. We currently cache the generated off_forcefield, so the same mechanism can be utilized for openmm_system.

The key/hash used for the cache must be selected with care. It must be insensitive to rotations or translations of the molecule, but should be sensitive to charge states, connectivity, and forcefield parameters. From @peastman:

One option for this is to serialize the System to XML with XmlSerializer.serialize(system) then compute a hash from the string. This will detect any change to the System or the Forces it contains, but will be unaffected by changes to particle positions.