WIP: McEventCollection clone using HepMC3::GenEvent

This merge request adds a new GenEventCollection class (DataVector<HepMC3::GenEvent>), which is a copy of the McEventCollection class (DataVector<HepMC::GenEvent>) except that the HepMC3 rather than HepMC2 classes are used. This code may not stay in the release long term, but it adds an explict dependency on the hepmc3 package, so that it will be installed under /cvmfs/atlas-nightlies.cern.ch as described in ATLINFR-3145. This will then allow others to develop using HepMC3 against the master nightlies.