Draft: ISF::TruthBinding - Fix build warning (ATLASSIM-7196)

ISF::TruthBinding::m_primaryTruthParticle is intended to point at the initial GenParticle passed to the simulation, as a way of keeping track of all the child particles caused by the same initial particle. This should be the same for all child particles produced and therefore should be a HepMC::ConstGenParticlePtr rather than a HepMC::GenParticlePtr. The fact that making this change introduced build failures indicated a deeper logic bug in the simulation code for jobs running multiple simulators (i.e. ATLFAST3)