For HepMC3, GenParticle barcodes can only be set once they are associated with a GenEvent

The changes in this merge request a a fix for simulation in HepMC3-based builds. Most changes are protected behind preprocessor checks, so would not be seen in HepMC2-based builds.

In order to be able to do this consistently with the HepMC2 implementation, it was necessary to extend the ITruthIncident interface to be able to retrieve a child particle barcode set by the simulator.

Tagging @averbyts, @ewelina, @mgignac, @jkretz, @dhirsch, @tlari, @mbandier for the other changes.
Tagging @lmijovic for the ITruthIncident changes.

Simulation/ISF/ISF_Core/ISF_Event/ISF_Event/ISFTruthIncident.h

@@ -86,6 +86,8 @@ namespace ISF {
     double                    childEkin(unsigned short index) const override final;
     /** Return the PDG Code of the i-th child particle */
     int                       childPdgCode(unsigned short index) const override final;
+    /** Return the barcode of the i-th child particle (if defined as part of the TruthIncident) otherwise return 0 */
+    Barcode::ParticleBarcode  childBarcode(unsigned short) const override final {return 0;};
     /** Return the i-th child as a HepMC particle type and assign the given
         Barcode to the simulator particle (usually only called for particles that
         will enter the HepMC truth event) */