Add `allow_nonintegral_charges` optional argument to enable user-provided nonintegral charges

[jchodera]

cc: https://github.com/openmm/openmm/issues/3678

There is a use case for adding an optional allow_nonintegral_charges argument that could be passed to ForceField.create_openmm_system in the OpenFF toolkit to permit users with manually-provided non-integral charges to still use this workflow.