Allow for other barostats in `SystemGenerator`

[hannahbaumann]

Currently, the barostat in _modify_forces is hard coded to be the MonteCarloBarostat (

openmmforcefields/openmmforcefields/generators/system_generators.py

Lines 323 to 324 in 839f225

	# TODO: Make sure we can support other kinds of barostats?
	barostat = openmm.MonteCarloBarostat(pressure, temperature, frequency)

).
For protein membrane simulations I want to use the MonteCarloMembraneBarostat. Would it be possible to allow for that here?

Additionally, it may be nice to throw a warning here if no box vectors are found and that it's switching the ff kwargs to nonperiodic:

openmmforcefields/openmmforcefields/generators/system_generators.py

Lines 384 to 385 in 839f225

	if topology.getPeriodicBoxVectors() is None:
	forcefield_kwargs.update(self.nonperiodic_forcefield_kwargs)

[mattwthompson]

_modify_forces reads as if nobody's ever used it with a different barostat. I suppose other barostats may have different function signatures so there's more bookkeeping needed to replace this line

openmmforcefields/openmmforcefields/generators/system_generators.py

Line 324 in 839f225

barostat = openmm.MonteCarloBarostat(pressure, temperature, frequency)

with something like barostat = self.barostat.__class__(*args) but that shouldn't be too hard?

(Why do we need to remake a new barostat instance here, anyway?)

[hannahbaumann]

I'm also not sure why we need to remake the barostat here. If it's not necessary, that would of course be easiest!

I wonder if something like this could work?

# Get the barostat
barostat = copy.deepcopy(self.barostat)
seed = np.random.randint(MAXINT)
barostat.setRandomNumberSeed(seed)
system.addForce(barostat)

[ijpulidos]

I don't think it's strictly needed, I read it as if we don't pass a barostat instance to the SystemGenerator initializer then it wouldn't add a barostat. That probably means that you need to add the barostat yourself after instantiating it and creating the system. Maybe that's the workaround.

That said, I think it would be convenient for the method to accept different classes/types of barostats there.