Provide mechanism to load/store GUI settings

When iterating on the development of some algorithm, it's common to repeatedly re-load Polyscope; in some cases, the user has to set the same options over and over again (e.g., the camera, the ground plane, the mesh material, etc.).

Would be nice to have a mechanism for saying, "save the state now" (e.g., upon exit) and also for reading this state back in upon the next invocation of Polyscope. E.g., functions storeGuiState( filename ) and loadGuiState( filename ).

Implementing these methods would entail some kind of serialization of the GUI state. Easiest would be to just handle "global" state that does not refer to any specific data passed to Polyscope.

Even cooler would be to also store settings attached to individual named data items—and ignore these settings if these items cannot be found (or have different types) on load. Polyscope should make it feasible to do this, since data can be queried by name.