Review and improve unit conversion in Gen/GenTune
The interface to RIVET library in Gen/GenTune relied on a feature of RIVET which automatically converted the HepMC event units from LHCb units [Mev, mm, ns] to units used by RIVET [GeV, mm, ns]. This feature seems to be disabled (at least in RIVET 3.1.4) so we decided not to rely on it any more, but always do the conversion in the interface (working on the deep-copy of the HepMC event). The code is already implemented in GenTune, only the logic and log messages need to be reviewed and updated.