add sensitiveIDs monitor

taking FTsensDetID as an exmaple:

thanks @zexu 's help

Sim/SimComponents/src/MCHitMonitor.cpp

@@ -57,6 +57,8 @@ private:
   IHistogram1D*              m_PMagHisto{nullptr};
   IHistogram1D*              m_NumHits{nullptr};
 
+  IHistogram1D* m_sensitiveIDs{nullptr};
+
   mutable std::vector<double> m_energyVec;
   mutable std::mutex          m_histogram_mutex;
 
@@ -114,6 +116,8 @@ void MCHitMonitor::initHistograms() {
 
   } // loop stations
 
+  m_sensitiveIDs = book1D( "Sensitive ID histo", -10, 16777215, 1000 ); // 16777215 =OxFFFF
+
   // p
   m_PMagHisto       = book1D( 4, "pMag", 0., 100, 200 );
   m_NumHits         = book1D( 1, "num hits", 0., 5000, 100 );
@@ -161,6 +165,9 @@ StatusCode MCHitMonitor::fillHistograms( const LHCb::MCHit* aHit ) const {
   // p
   m_PMagHisto->fill( aParticle->p() / Gaudi::Units::GeV );
 
+  // sensitive IDs
+  m_sensitiveIDs->fill( aHit->sensDetID() );
+
   // average of entrance and exit...
   Gaudi::XYZPoint mcHitPoint = aHit->midPoint();