Replace GeantImporter with GeantPhysicsParams to support physics onload

[sethrj]

Using VolumeParams+GeantGeoParams as an archetype, we should add Geant4->Celeritas mappings through a GeantPhysicsParams class:

Geant4	Celeritas
ParticleDefinition	ParticleId
Material	GeoMatId
MaterialCutsCouple	PhysMatId
MaterialPropertiesTable	OptMatId
OpticalSurface	PhysSurfId
VProcess	ProcessId

• For physics "onload" this lets us map processes/models from ActionID back to a thread-local Geant4 process. (This is the first and most important thing to add.) See https://github.com/celeritas-project/celeritas/issues/1898
• Replaces the particle ID track reconstruction used by the hit processor.
• Builds inp::Particles from Geant4 data based on the user offload requirements.
• Builds physics inp as well?
• Replaces ext/GeoOpticalIdMap