MCTruth: use non-const particles
This is similar to !58354 (merged) and removes unnecessary const
qualifiers on
GenParticle
and ISFParticle
to avoid const_cast
downstream for
the following classes:
PrimaryParticleInformation
TrackBarcodeInfo
TrackInformation
VTrackInformation
This also reduces the amount of #ifdef HEPMC3
code.