message: parsing also HEPmc event header
diff: #include "AliGenCocktailEventHeader.h"
  #include "AliGenEventHeader.h"
  #include "AliCollisionGeometry.h"
  +#include "AliGenHepMCEventHeader.h"
  #include "AliEventPoolManager.h"
  #include "AliBasicParticle.h"
  @@ -1465,13 +1466,17 @@ Double_t AliAnalysisTaskPhiCorrelations::GetCentrality(AliVEvent* inputEvent, TO
  }
  AliCollisionGeometry* collGeometry = dynamic_cast<AliCollisionGeometry*> (eventHeader);
  - if (!collGeometry)
  + AliGenHepMCEventHeader* hepMCHeader = dynamic_cast<AliGenHepMCEventHeader*> (eventHeader);
  + if (!collGeometry && !hepMCHeader)
  {
  eventHeader->Dump();
  AliFatal("Asking for MC_b centrality, but event header has no collision geometry information");
  }
  + if (collGeometry)
  centrality = collGeometry->ImpactParameter();
  + else if (hepMCHeader)
  + centrality = hepMCHeader->impact_parameter();
  }
  else if (fCentralityMethod == "MCGen_V0M")
  {
  