Parse atomic charges in `read_gaussian_out`

Closes #1066 (closed)

This MR adds functionality to read Mulliken, Löwdin, or Hirshfeld charges and spin charges in read_gaussuan_out. Gaussian logs checked are attached H2O.zip O2.zip. I also confirmed pytest calculator/gaussian --calculators=gaussian -v passes.

By default, Gaussian prints Mulliken charges, which is stored in atoms.calc.results['charges']. When an option to print Löwdin or Hirshfeld charges is set, they overwrites atoms.calc.results['charges'].

Checklist

• I am familiar with ASE's contribution guidelines.
• Doc strings in code changed in this MR are up to date.
• Unit tests have been added for new or changed code.
• Issue is resolved via "closes #XXXX" if applicable.