Introduce a common naming scheme for methods and variables related to GenParticles representing particles after all interactions so far

The Current GenParticle represents the state of particle currently being simulated after its most recent interaction. Initially it is the same as the "Generation Zero" GenParticle (representing the initial state of a particle prior to any interactions), but is updated each time the particle interacts and survives.

This merge request introduces a common naming scheme for methods of G4 UserInformation classes and ISF::TruthBinding and variables in client code relating to the Current GenParticle.

This makes it easier to see how the various UserInformation classes are kept in sync.