Allow histograms to be saved in custom directories

Before this change, RootHistogramSink always puts a histogram
in a top level directory named according to the component name.
This change allows a histogram to be saved in a custom directory
by naming it with a leading slash. In that case, the component
name is ignored.

GaudiCommonSvc/src/HistogramPersistencySvc/RootHistogramSink.cpp

@@ -67,6 +67,11 @@ namespace {
     auto totNBins = ( ( axis[index].nBins + 2 ) * ... );
     assert( weights.size() == totNBins );
 
+    if ( name[0] == '/' ) {
+      dir  = "";
+      name = name.substr( 1 );
+    }
+
     // take into account the case where name contains '/'s (e.g. "Group/Name") by
     // moving the prefix into dir
     if ( auto pos = name.rfind( '/' ); pos != std::string::npos ) {

GaudiExamples/src/Histograms/CounterHistos.cpp

@@ -90,6 +90,7 @@ namespace Gaudi {
 
           // updating histograms
           ++m_gauss[gauss];
+          ++m_gaussAbsName[gauss];
           ++m_gaussVflat[{ flat, gauss }];
           ++m_gaussVflatVgauss[{ flat, gauss, gauss2 }];
           ++m_gauss_noato[gauss];
@@ -273,6 +274,10 @@ namespace Gaudi {
         mutable Accumulators::LogHistogram<1> m_log_gauss{ this, "LogGauss", "Log, Gaussian", { 5, 0, 2 } };
         mutable Accumulators::LogHistogram<2, Accumulators::atomicity::full, float> m_log_gaussVflat{
             this, "LogGaussFlat", "LogLog, Gaussian V Flat", { { 5, 0, 2 }, { 5, 0, 2 } } };
+
+        // Histogram in an absolute location
+        mutable Gaudi::Accumulators::Histogram<1> m_gaussAbsName{
+            this, "/TopDir/SubDir/Gauss", "Gaussian mean=0, sigma=1, atomic", { 100, -5, 5, "X" } };
       };
       DECLARE_COMPONENT( GaudiHistoAlgorithm )
     } // namespace Counter