Introduce a common naming scheme for methods and variables related to GenParticles representing particles prior to any interactions

The "Generation Zero" GenParticle is a GenParticle representing the intial state of the particle currently being simulated (i.e. prior to any interactions). Generation Zero GenParticles may be primary particles (initial truth particles passed to the simulation (i.e. those whose parent particles are not simulated)) or secondary particles (new particles created by interactions and decays during the simulation). This merge request introduces a common naming scheme for methods of G4 UserInformation classes and ISF::TruthBinding and variables in client code relating to Generation Zero GenParticles.

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