Implement framework for optical physics models

[hhollenb]

This is an initial implementation of optical models and their associated data structures.

General overview:

• OpticalModel: acts like both a process (building cross sections / mean free paths) and as a model (action to run the interactor).
• ImportedOpticalMaterials: physics data for optical models is really just optical material properties. This serves as a list of imported optical material data that models will access directly to get their physics data.
• OpticalModelBuilder: constructs the ImportedOpticalMaterials and is responsible for building the appropriate models based on the user's selection.
• OpticalPhysicsParams: responsible for building each optical model, adding them to the action registry, and building their MFP grids in OpticalPhysicsParamsData.
• OpticalMfpBuilder: A helper class to have optical models build MFP grids on a per optical material basis.
• AbsorptionModel: An almost trivial optical model. Has no data and just kills the track when called.
• RayleighModel: A more complex optical model. Included to make sure API for building model-specific data is sensible.

This PR isn't merge ready yet, and is meant to be a discussion point for how we want to organize optical models. Some functions and classes are empty, and I'm in the middle of adding tests.

Thoughts / discussion questions:

• May be better to have something like this in ImportData:

  std::unordered_map<GeoMatIdx, ImportOpticalMaterialId> geo_optical_map;
  std::vector<ImportOpticalMaterial> optical_materials;

• Will we want an OpticalMaterialParams? Is there ever a point where we need to process ImportOpticalMaterials and store the data outside of optical model specific data collections?
• How much flexibility do we want to give to user defined optical models? Should we have an ImportOpticalModel structure? A lambda table in ImportOpticalModel would duplicate the mean free paths in ImportOpticalMaterial...
• OpticalMfpBuilder is a bit awkward. There should be a better way to do this that is also portable to Cerenkov and scintillation parameter building.

[sethrj]

Does this supersede #1162 ?

[hhollenb]

Does this supersede #1162 ?

Yes, I'll close that one. I reused any relevant code but had to make enough major changes I thought it was worthwhile to start from a fresh branch.