hepmc3 for Debian

By default the HepMC3 is built with ROOT IO support, however, as the 
root6 package is not available in Debian, this version provides no such 
support.

 -- HepMC3 Developers <hepmc-dev@NOSPAM.cern.ch>   Mon, 25 Feb 2019 13:53:32 +0000