Partial, formal, and total charges are reported with inconsistent units

[jchodera]

The current version of the toolkit reports partial, formal, and total charges with inconsistent units:

>> from openforcefield.topology import Molecule
>>> molecule = Molecule.from_smiles('c1ccccc1')
>>> molecule.total_charge
0
>>> molecule.atoms[0].formal_charge
0
>>> molecule.atoms[0].partial_charge
Quantity(value=0.0, unit=elementary charge)
>>> molecule.partial_charges
Quantity(value=array([0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0.]), unit=elementary charge)

Instead, we should presumably adopt a consistent output format (with or without units) for all charge-related quantities.

[j-wags]

Good catch. #471 will standardize to report partial and formal charges in units of elementary charge.

[j-wags]

I'm also making the setter for Atom.formal_charge accept int or Quantity, since there's a good argument for both. All getters will return Quantity, and all other setters will require Quantity.