Add a mapping save and load from mapping functions

[jennyfothergill]

In any workflow that might actually use grits, we would need to be able to save and reload the mapping:

1. Create CG_Compound
2. Save to simulation input format (e.g., gsd)
3. Run simulation
4. Load new structure from simulation output
5. Load mapping
6. Backmap!

[mikemhenry]

A quick thought:

If performance is important, then a binary format would be better than text!