Introduce a common naming scheme for methods and variables related to primary GenParticles

Primary GenParticles represent the initial truth particles passed to the simulation (i.e. those whose parent particles are not simulated). This merge request introduces a common naming scheme for methods of G4 UserInformation classes and ISF::TruthBinding and variables in client code relating to Primary GenParticles.

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