Hepmc3 nightly fixes 04122020 part 7

In this MR:

• Make xAODHepMCtool compatible with HePMC3

This MR contains also !38878 (merged) Therefore either 38878 should be merged before this MR, or this MR should be merged alone.

Generators/AtlasHepMC/AtlasHepMC/GenParticle.h

@@ -49,7 +49,7 @@ inline GenParticlePtr newGenParticlePtr(const HepMC::FourVector &mom = HepMC::Fo
 inline int barcode(GenParticle p) {   return    p.barcode(); }
 template <class T> inline int barcode(T p) {   return    p->barcode(); }
 template <class T> bool suggest_barcode(T p, int i) {return p->suggest_barcode(i);}
-inline bool suggest_barcode(GenParticle p, int i) {return p.suggest_barcode(i);}
+inline bool suggest_barcode(GenParticle& p, int i) {return p.suggest_barcode(i);}
 template <class T> bool suggest_barcode(T* p, int i) {return p->suggest_barcode(i);}
 namespace Print {
 inline void line(std::ostream& os,const GenParticle& p) {p.print(os);}