Clean and DRY nonbonded potential code

[mcwitt]

In the initial implementation of NonbondedInteractionGroup in #578 (and previously in #542), we introduced duplicate code by copying and modifying the original nonbonded potential implementation. We should revisit this at some point to DRY the common parts, and apply some fixes suggested in the above PRs that apply in multiple places.

Miscellaneous items applying to code that currently exists in multiple nonbonded potentials:

• Add NonbondedInteractionGroup potential, rename existing nonbonded potentials #578 (comment)
• Add NonbondedInteractionGroup potential, rename existing nonbonded potentials #578 (comment)
• Add NonbondedInteractionGroup potential, rename existing nonbonded potentials #578 (comment)
• Add NonbondedInteractionGroup potential, rename existing nonbonded potentials #578 (comment)
• Add checks for invalid nonbonded params (e.g. negative sigma or epsilon) Wrap NonbondedAllPairs, NonbondedPairList #643 (comment)
• Allow computing all-pairs potential on subset of atoms, add consistency check #660 (comment)
• Allow computing all-pairs potential on subset of atoms, add consistency check #660 (comment)