Pass run start time to the digitizers. For STS, start noise simulation at the run start time.

In the STS digitizer, noise is simulated from the previous event time to the current event time. For the first event, noise was simulated from t = 1000 ns (hard-coded) to the first event time.

Since the run start time (default 1000 ns) can be changed by the user, comparison of results with different run start times was difficult.

This MR passes the run start time to the digitizers, and for the STS, the noise simulation now starts at the specified run start time.

core/base/CbmDigitizeBase.h

@@ -170,10 +170,17 @@ public:
   void SetProduceNoise(Bool_t choice = kTRUE) { fProduceNoise = choice; }
 
 
+  /** @brief Set the run start time
+   ** @param Run start time [ns]
+   **/
+  void SetRunStartTime(Double_t time) { fRunStartTime = time; }
+
+
 protected:
   Bool_t fEventMode;                          /// Flag for event-by-event mode
   Bool_t fProduceNoise;                       /// Flag for production of inter-event noise
   Bool_t fCreateMatches;                      /// Flag for creation of links to MC
+  Double_t fRunStartTime = 0;                 /// Start time of run [ns]
   Int_t fCurrentInput;                        /// Number of current input
   Int_t fCurrentEvent;                        /// Number of current MC event
   Int_t fCurrentMCEntry;                      /// Number of current MC entry