ThePEG now runs with either HepMC2 or HepMC3 (thanks to Andrii
Verbytskyi). The selection is done with the --with-hepmcversion flag
in configure. The HepMC3File class (and friends) should no longer
be needed. Also HIHepMCFile is no longer needed as output of heavy ion
info is controlled by a switch in HepMCFile. The obsolete classes are
still there but should be removed soon.