message: Using AliExternalTrackParam to get momentum
diff: #include "AliAnalysisTaskAO2Dconverter.h"
  -#include "AliAnalysisTaskAO2Dconverter.h"
  +#include "AliExternalTrackParam.h"
  void read()
  {
  const Char_t* name = AliAnalysisTaskAO2Dconverter::TreeName[AliAnalysisTaskAO2Dconverter::kTracks];
  ROOT::RDataFrame d(name, "AO2D.root");
  ROOT::RDF::TH2DModel betamodel("beta", ";#it{p} (GeV/#it{c});TOF #beta;", 1000, 0.1, 5, 1000, 0, 2);
  - auto h = d.Define("beta", "fTOFsignal > 0.1 ? fLength/(fTOFsignal * (TMath::C() * 1.e2 / 1.e12)) : -10").Histo2D(betamodel, "fTPCinnerP", "beta");
  + auto h = d.Define("param", "std::array<double, 5> p{fY, fZ, fSnp, fTgl, fSigned1Pt}; return p;")
  + .Define("covar", "std::array<double, 15> p{fCYY, fCZY, fCZZ, fCSnpY, fCSnpZ, fCSnpSnp, fCTglY, fCTglZ, fCTglSnp, fCTglTgl, fC1PtY, fC1PtZ, fC1PtSnp, fC1PtTgl, fC1Pt21Pt2}; return p;")
  + .Define("P", "AliExternalTrackParam((double)fX, (double)fAlpha, param.data(), covar.data()).GetP()")
  + .Define("beta", "fTOFsignal > 0.1 ? fLength/(fTOFsignal * (TMath::C() * 1.e2 / 1.e12)) : -10")
  + .Histo2D(betamodel, "P", "beta");
  new TCanvas();
  gPad->SetLogz();
  gPad->SetLogx();
  