Use HepMC virtualization interface in Gen/GenTune
In view of changing the major version of the HepMC library used by Gauss, the Gen/GenTune code needs to updated to use a virtualization layer should such a layer be implemented in Gauss/LHCb. Such an attempt or virtualization interface for HepMC is already implemented in RIVET 3.x, but we'd like to avoid depending on it in order to limit the level of inter-dependence.