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