HepMC3 event record library: pythia6 -> include Relation

HepMC3 event record library

### pythia6 → include Relation

File in interfacespythia6 | Includes file in include
--- | ---
include / Pythia6 / Pythia6ToHepMC3.cc | HepMC3 / Attribute.h
include / Pythia6 / Pythia6ToHepMC3.cc | HepMC3 / GenEvent.h
include / Pythia6 / Pythia6ToHepMC3.cc | HepMC3 / GenRunInfo.h
include / Pythia6 / Pythia6ToHepMC3.cc | HepMC3 / HEPEVT_Wrapper_Template.h
include / Pythia6 / Pythia6ToHepMC3.cc | HepMC3 / Print.h
include / Pythia6 / Pythia6ToHepMC3.cc | HepMC3 / Writer.h
include / Pythia6 / Pythia6ToHepMC3.cc | HepMC3 / WriterAscii.h
include / Pythia6 / Pythia6ToHepMC3.cc | HepMC3 / WriterAsciiHepMC2.h
include / Pythia6 / Pythia6ToHepMC3.cc | HepMC3 / WriterHEPEVT.h
include / Pythia6 / Pythia6ToHepMC3.cc | HepMC3 / WriterPlugin.h