add charge embedding

[loriab]

there's a partial embedding test that'll fail b/c incomplete.

• add charge_embedding part of the schema, store on Mol
  • let the qcengine computer run it through psi4 since not standardized in qcengine and per-qc-prog
  • add a basic test for it
  • fix its output and qcvars
• broaden tests to include heterogenous fragments (different elements, different frag lengths) and charges on fragments and (on my side) beyond energies

[codecov]

Codecov Report

Merging #15 (d7c2527) into main (b8f0645) will increase coverage by 1.80%.
Report is 13 commits behind head on main.
The diff coverage is 71.84%.

Additional details and impacted files