Basic muon decay physics

[sethrj]

Add decay for muons.

Possible design:

• new DecayProcess class (maybe eventually we should rename Process to InteractionProcess?) that applies to one or more particle IDs

• Decay process vector added to PhysicsParams

• decay constants from particle types get turned into effective cross sections inside calc_physics_step_limit

• select decay process as part of select_discrete_interaction?

• Do we have multiple "models" for the process? Or separate decay channels that should be sampled? Do we ever need to have different models per physics region?

• Otherwise like optical physics we could have single model per process, and if so DecayProcess should probably inherit from CoreStepActionInterface

• G4VDecayChannel: G4MuonDecayChannel