cmake file for HepMC3
Hi @akraszna ,
this MR adds a cmake file for HepMC3.
HepMC3 itself has cmake configuration files, but as far as I understand one should use the LCG style cmake files. Therefore the added file is basically a copy of FindHepMC.cmake with changed name and added "search" component. Also, I'm not sure about the RPM. The RH/Fedora packages have multiple RPMS, but here I've copypasted from FindHepMC.cmake and wrote only one.