HepMC3 event record library: Pythia6 -> include Relation

HepMC3 event record library

### Pythia6 → include Relation

File in interfacespythia6includePythia6 | Includes file in include
--- | ---
Pythia6ToHepMC3.cc | HepMC3 / Attribute.h
Pythia6ToHepMC3.cc | HepMC3 / GenEvent.h
Pythia6ToHepMC3.cc | HepMC3 / GenRunInfo.h
Pythia6ToHepMC3.cc | HepMC3 / HEPEVT_Wrapper_Template.h
Pythia6ToHepMC3.cc | HepMC3 / Print.h
Pythia6ToHepMC3.cc | HepMC3 / Writer.h
Pythia6ToHepMC3.cc | HepMC3 / WriterAscii.h
Pythia6ToHepMC3.cc | HepMC3 / WriterAsciiHepMC2.h
Pythia6ToHepMC3.cc | HepMC3 / WriterHEPEVT.h
Pythia6ToHepMC3.cc | HepMC3 / WriterPlugin.h