rework force field assignment and fix MSONable bugs

[orionarcher]

This PR is intended to make a few updates and changes to the OpenMM IO infrastructure.

Namely

• [x] adding support for charge scaling with Sage
• [x] fixing a broken charge scaling test
• [x] modify classes to make them MSONable
  • [x] OpenMMSet
  • [x] OpenMMInputGenerator
  • [x] InputFiles
• [ ] separate OpenMMSageGenerator from OpenMMGenerator