Add method to save a simulation's reference values and units to file

[chrisjonesBSU]

Right now, saving the information from a simulation to restart later (or use in a different simulation method such as tensile testing) is pretty easy since we can save both the state (gsd file) and forces (pickle) from a simulation. Really, we also need a way to save information about the simulation's reference values and units to file as well. In the simulation class, these are stored as a dictionary, so we can use pickles, npy files, json files, etc... I think we should pick one option for saving these, and add a save_reference_values method to Simulation.