Preparatory changes for the HepMC3 migration for packages in AthSimulation

This merge request mainly makes the following replacements for classes which are part of the AthSimulation project:

HepMC::GenParticle* --> HepMC::GenParticlePtr
const HepMC::GenParticle* --> HepMC::ConstGenParticlePtr
HepMC::GenVertex* --> HepMC::GenVertexPtr
const HepMC::GenVertex* --> HepMC::ConstGenVertexPtr

Plus a few other syntax replacements to use helper functions defined in AtlasHepMC/GenParticle.h and AtlasHepMC/GenVertex.h. The purpose is to prepare for the migration from HepMC2 to HepMC3. Further syntax changes will be required.