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

@@ -50,7 +50,7 @@ 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);}
 //Smart pointers should not be used with HepMC2. But it happens.
-template <class T> bool suggest_barcode(std::unique_ptr<T>& p, int i) {return p->suggest_barcode(i);}
+inline bool suggest_barcode(std::unique_ptr<HepMC::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 {