Add printing overloads for non-const GenParticlePtr and GenVertexPtr objects
While debugging Simulation in the master--HepMC3 nightly, it was noticed that for (non-const) GenParticlePtr and GenVertexPtr instances the bare-pointer was being printed out, while for ConstGenParticlePtr and ConstGenVertexPtr instance the object pointed to by the shared_ptr was printed out. This merge request adds the missing functions for the non-const objects. NB The changes in the MR are behind an #ifdef HEPMC3 so will not be picked up by the CI. (In the future it may be beneficial to move this functionality into HepMC3/PrintStreams.h.)