Creating charge constraints is a hassle

[lilyminium]

Expected behavior

In the scenario that I know which molecules and atom indices I want to create charge constraints for, it's kind of a hassle to make them; I need to use Molecule.get_atoms. It would be nice to have a Molecule.atoms array to index.

Actual behavior

To Reproduce

import psiresp
from psiresp.tests.datafiles import DMSO

...

Screenshots

Computing environment:

• Operating system (Linux, Mac, Windows?):
• Python version (3.7, 3.8, 3.9?):
• Version of PsiRESP:
• Output of running conda list:

Additional context