Allow for SMIRNOFFTemplateGenerator to take a list of smirnoff force fields #422
Description
Currently the SMIRNOFFTemplateGenerator takes in a string, as either the filename for an offxml or a serialized offxml.
The ask is to allow forcefield: str | list[str] | None, where https://github.com/openmm/openmmforcefields/blob/main/openmmforcefields/generators/template_generators.py#L1386 would just unpack the list and pass it all as arguments to create the one openmm.toolkit.ForceField.