fix dead time correction for Red/Green (period) NeXus IDF V2 data

Since the Red/Green mode handling was added for NeXus HDF5 IDF V2, the
histogram numbers of the routed histograms carry the period offset, i.e.
histoNo = period*10000 + histo. PRunBase::DeadTimeCorrection() was still
addressing the dead time parameter vector with 'histoNo-1', hence for any
period > 0 it read far beyond the end of the vector. The garbage picked up
there scaled the counts, so every invocation of musrfit gave a different
(sometimes nan) result for the affected run blocks.

The dead time parameters are stored contiguously, period-by-period, and
therefore need to be addressed as period*noOfHistosPerPeriod + histo - 1.
Introduce PERIOD_HISTO_OFFSET for the period encoding, keep the number of
histos per period in PRawRunData, and add PRawRunData::GetDeadTimeParam(histoNo)
which does the mapping and guards against out-of-range access.

Cross-checked against the HDF4 IDF V1 twin of the same run, where the two
periods are flattened into histos 1-192: both readers now yield bit-identical
chisq, with and without dead time correction.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/include/PMusr.h

@@ -264,6 +264,15 @@
 /// Offset added to function indices for parameter parsing
 #define MSR_PARAM_FUN_OFFSET 20000
 
+//-------------------------------------------------------------
+/**
+ * <p>Offset used to encode the period (Red/Green mode) into the histogram
+ * number, i.e. histoNo = period*PERIOD_HISTO_OFFSET + histo, with histo
+ * starting at 1 and period starting at 0. Currently used for NeXus IDF V2
+ * (ISIS) data which are organized as counts[period][histo][bin].
+ */
+#define PERIOD_HISTO_OFFSET 10000
+
 //-------------------------------------------------------------
 /**
  * <p>Fourier transform unit tags.
@@ -880,7 +889,9 @@ class PRawRunData {
     virtual const Bool_t DeadTimeCorrectionReady();
     virtual const Int_t GetNumberOfGoodFrames() { return fNumberOfGoodFrames; }
     virtual const std::vector<float> GetDeadTimeParam() { return fDeadTimeParam; }
+    virtual const Double_t GetDeadTimeParam(const UInt_t histoNo);
     virtual const UInt_t GetNoOfHistos() { return fData.Size(); }
+    virtual const UInt_t GetNoOfHistosPerPeriod() { return fNoOfHistosPerPeriod; }
     virtual PRawRunDataSet* GetDataSet(const UInt_t idx, Bool_t wantHistoNo = true);
     virtual const PDoubleVector* GetDataBin(const UInt_t histoNo) { return fData.GetData(histoNo); }
     virtual const PNonMusrRawRunData* GetDataNonMusr() { return &fDataNonMusr; }
@@ -923,6 +934,7 @@ class PRawRunData {
     virtual void SetRedGreenOffset(PIntVector &ivec) { fRedGreenOffset = ivec; }
     virtual void SetNumberOfGoodFrames(Int_t ival) { fNumberOfGoodFrames = ival; }
     virtual void SetDeadTimeParam(std::vector<float> dvec) { fDeadTimeParam = dvec; }
+    virtual void SetNoOfHistosPerPeriod(UInt_t ival) { fNoOfHistosPerPeriod = ival; }
     virtual void SetDataSet(PRawRunDataSet &dataSet, UInt_t idx=-1) { fData.Set(dataSet, idx); }
 
     PNonMusrRawRunData fDataNonMusr; ///< keeps all ascii- or db-file info in case of nonMusr fit
@@ -964,6 +976,7 @@ class PRawRunData {
     PIntVector fRedGreenOffset;             ///< keeps the Red/Green offsets
     std::vector<float> fDeadTimeParam;      ///< dead time parameter vector needed for pulsed sources
     Int_t fNumberOfGoodFrames{0};           ///< needed to correct dead times at pulsed sources
+    UInt_t fNoOfHistosPerPeriod{0};         ///< number of histos per period (Red/Green mode); 0 means the run has no periods
 
     PRawRunDataVector fData;                ///< keeps the histos together with the histo related properties such as T0, first good bin, etc.
 };

src/include/PRunDataHandler.h

@@ -876,6 +876,8 @@ Bool_t PRunDataHandler::ReadNexusFileIdf2(T& nxs_file)
     noOfPeriods = dims[0];
     noOfHistos = dims[1];
     histoLength = dims[2];
+    // needed to properly address period related information like the dead time parameters
+    runData.SetNoOfHistosPerPeriod(noOfHistos);
     // get all necessary attributes
     if (count_ds.HasAttribute("t0_bin"))
       t0_bin = std::any_cast<int>(count_ds.GetAttribute("t0_bin"));
@@ -893,7 +895,7 @@ Bool_t PRunDataHandler::ReadNexusFileIdf2(T& nxs_file)
     for (int i=0; i<noOfPeriods; i++) {
       for (int j=0; j<noOfHistos; j++) {
         dataSet.Clear();
-        dataSet.SetHistoNo(i*10000+j+1); // i.e. histo numbers start with 1
+        dataSet.SetHistoNo(i*PERIOD_HISTO_OFFSET+j+1); // i.e. histo numbers start with 1
         dataSet.SetTimeZeroBin(t0_bin);
         dataSet.SetFirstGoodBin(fgb);
         dataSet.SetLastGoodBin(lgb);