Add convenience method to Molecule to return whether user has provided partial charges

[jchodera]

Is your feature request related to a problem? Please describe.
Currently, there's no reliable way to determine whether a user has set partial charges for a Molecule object. We can check if the charges are all zero, but that doesn't necessarily mean the user hasn't set them all to zero. If we had a simple API for determining whether user-specified charges are present, we could simplify choices about whether or not to use charges from user-provided molecules.

Describe the solution you'd like
We will have to carefully think through the implications, but one simple approach could be

• Molecule charge fields are None if not defined (e.g. because the molecule was programmatically constructed or read from an SDF file that lacks charges)
• Molecule.partial_charges is not None would be a simple idiom for determining whether charges are present

Describe alternatives you've considered
Currently, the only way to tell if user-specified charges may be present is to check if all charges are zero---something that is not guaranteed to produce the desired result and for which we do not even have a convenience function.

Additional context
I'm trying to determine how we should indicate that user-provided charges should be used in openmm-forcefields: openmm/openmmforcefields#88