Replace const HepMC::GenParticlePtr with HepMC::ConstGenParticlePtr

This is just a small set of changes to ensure that the meaning of some lines isn't changed by the switch to using the HepMC::GenParticlePtr typedefs (part of the preparation for the HepMC3 migration).

For HepMC2

const HepMC::GenParticlePtr == HepMC::GenParticle* const
HepMC::ConstGenParticlePtr == const HepMC::GenParticle*

FYI @averbyts