Re-add get1DVarHist back into samplePDFFDBase

[dbarrow257]

Pull request description

During the previous merge, get1DVarHist was dropped. This was likely because of getting interaction mode information back from the ReturnKinematicVariable[ByReference] as it returns a double and mode was defined as an integer. Add this function back in whilst also defining the getNSamples and getNMCEvent functions.

Changes or fixes

Examples

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[KSkwarczynski]

What is ChannelIndex?

[dbarrow257]

In FD code, there is the concept of oscillation channels. Each oscillation channel has an associated FDMCStruct object. These channels' indexing depends entirely on the ordering defined in the sample YAML.

If you want to loop over each oscillation channel and make an event rate breakdown by oscChan, you need some way to select events in oscillationChannel j (j=6, for example). To do this from ReturnKinematicParameter, we need to store the index of the oscillation channel somewhere in it's memory, thus ChannelIndex.

You can get name of the j'th oscillationChannel index from FDMCStruct.flavourName

[dbarrow257]

In DUNE code for example:

const double* samplePDFDUNEAtm::GetPointerToKinematicParameter(KinematicTypes KinPar, int iSample, int iEvent) {
  double* KinematicValue;


  switch (KinPar) {
  case kTrueNeutrinoEnergy:
    KinematicValue = &(dunemcSamples[iSample].rw_etru[iEvent]);
    break;
  case kRecoNeutrinoEnergy:
    KinematicValue = &(dunemcSamples[iSample].rw_erec[iEvent]);
    break;
  case kTrueCosZ:
    KinematicValue = &(dunemcSamples[iSample].rw_truecz[iEvent]);
    break;
  case kRecoCosZ:
    KinematicValue = &(dunemcSamples[iSample].rw_theta[iEvent]);
    break;
  case kOscChannel:
    KinematicValue = &(MCSamples[iSample].ChannelIndex);
    break;
  case kMode:
    KinematicValue = MCSamples[iSample].mode[iEvent];
    break;
  default:
    MACH3LOG_ERROR("Unknown KinPar: {}",KinPar);
    throw MaCh3Exception(__FILE__, __LINE__);
  }
  
  return KinematicValue;
}