Add support for multi-electron reduction

[WardLT]

We are going to start moving towards multi-electron redox:

• Add ability to MoleculeData record to express "double reduced" and "double oxidized"
• Double-check that QCEngine can detect whether something is doublet or triplet (@ndandu, help!)
• Make the "redox recipes" work for these
• Run some initial molecules from the Minteer and Sigman groups to verify NWChem can compute these correctly