HepMcParticleLink constructor syntax clean-up.

Avoid using 0 in combination with HepMcParticleLink::IS_EVENTNUM in constructor calls as this is not properly supported. It is more correct to directly use 0 with HepMcParticleLink::IS_POSITION to indicate that the link is to the first GenEvent in the McEventCollection.

(Relates to ATLASSIM-6999)

Simulation/ISF/ISF_Core/ISF_Event/src/ISFTruthIncident.cxx

@@ -210,7 +210,7 @@ HepMC::GenParticlePtr ISF::ISFTruthIncident::updateHepMCTruthParticle( ISF::ISFP
     newHMPL = new HepMcParticleLink(hepTruthParticle, oldHMPL->eventIndex(), HepMcParticleLink::IS_EVENTNUM);
     delete oldHMPL;
   } else {
-    newHMPL = new HepMcParticleLink(hepTruthParticle, 0, HepMcParticleLink::IS_EVENTNUM); // FIXME should be HepMcParticleLink::IS_POSITION
+    newHMPL = new HepMcParticleLink(hepTruthParticle, 0, HepMcParticleLink::IS_POSITION);
   }
   particle.setParticleLink(newHMPL);