message: GFWFlow running with AMPT impact parameters, need to test with eg HIJING
diff: @@ -33,6 +33,7 @@ Extention of Generic Flow (https://arxiv.org/abs/1312.3572)
  #include <vector>
  #include "AliCollisionGeometry.h"
  #include "AliGenEventHeader.h"
  +#include "AliGenHepMCEventHeader.h"
  ClassImp(AliAnalysisTaskGFWFlow);
  @@ -292,30 +293,22 @@ void AliAnalysisTaskGFWFlow::UserCreateOutputObjects(){
  // mywatchStore.Reset();
  // mywatch.Start(kTRUE);
  };
  +/*
  +AliMCEvent* ev = dynamic_cast<AliMCEvent*>(MCEvent());
  +if(!ev) { AliFatal("MC event not found!"); return 0; }
  +AliCollisionGeometry* headerH;
  +headerH = dynamic_cast<AliCollisionGeometry*>(ev->GenEventHeader());
  +impactParameter = headerH->ImpactParameter();
  +*/
  AliMCEvent *AliAnalysisTaskGFWFlow::FetchMCEvent(Double_t &impactParameter) {
  if(!fIsTrain) { AliFatal("Snap, Jim! Ain't no train here!\n"); return 0; }
  AliMCEvent* ev = dynamic_cast<AliMCEvent*>(MCEvent());
  if(!ev) { AliFatal("MC event not found!"); return 0; }
  - AliGenEventHeader *header = dynamic_cast<AliGenEventHeader*>(ev->GenEventHeader());
  + AliGenHepMCEventHeader
  + *header = dynamic_cast<AliGenHepMCEventHeader*>(ev->GenEventHeader());
  if(!header) { AliFatal("MC event not generated!"); return 0; }
  - AliCollisionGeometry* headerH;
  - TString genName;
  - TList *ltgen = (TList*)ev->GetCocktailList();
  - if (ltgen) {
  - for(auto&& listObject: *ltgen){
  - genName = Form("%s",listObject->GetName());
  - if (genName.Contains("Hijing")) {
  - headerH = dynamic_cast<AliCollisionGeometry*>(listObject);
  - break;
  - }
  - }
  - }
  - else
  - headerH = dynamic_cast<AliCollisionGeometry*>(ev->GenEventHeader());
  - if(headerH){
  - impactParameter = headerH->ImpactParameter();
  - }
  + impactParameter = header->impact_parameter();
  return ev;
  }
  @@ -324,7 +317,7 @@ void AliAnalysisTaskGFWFlow::UserExec(Option_t*) {
  Double_t lImpactParameter = -1;
  AliMCEvent *fEv = FetchMCEvent(lImpactParameter);
  if(!fEv) return;
  - if(lImpactParameter < 0) AliFatal("Impact parameter is negarive!\n");
  + if(lImpactParameter < 0) AliFatal("Impact parameter is negative!\n");
  Double_t l_Cent = GetCentFromIP(lImpactParameter);
  fMultiDist->Fill(l_Cent);
  if(l_Cent>70 || l_Cent<5) return; //not considering anything below 5% or above 70%
  