TPC digi: Cut digits arriving before timeframe/readout start

Relates to
https://its.cern.ch/jira/browse/O2-5395

and should allow to treat correctly events coming before the timeframe
start to reduce the startup effect in MC.

Detectors/TPC/simulation/include/TPCSimulation/SAMPAProcessing.h

@@ -258,6 +258,10 @@ inline float SAMPAProcessing::getZfromTimeBin(float timeBin, Side s) const
 
 inline TimeBin SAMPAProcessing::getTimeBinFromTime(float time) const
 {
+  if (time < 0.f) {
+    // protection and convention for negative times (otherwise overflow)
+    return 0;
+  }
   return static_cast<TimeBin>(time / mEleParam->ZbinWidth);
 }
 

Detectors/TPC/simulation/src/Digitizer.cxx

@@ -111,6 +111,12 @@ void Digitizer::process(const std::vector<o2::tpc::HitGroup>& hits,
         }
         const float absoluteTime = eleTime + mTDriftOffset + (mEventTime - mOutputDigitTimeOffset); /// in us
 
+        /// the absolute time needs to be within the readout limits
+        /// (otherwise negative times would all be accumulated in the 0-th timebin further below)
+        if (!(absoluteTime >= 0 /* && absoluteTime <= timeframelength */)) {
+          continue;
+        }
+
         /// Attachment
         if (electronTransport.isElectronAttachment(driftTime)) {
           continue;
@@ -217,9 +223,14 @@ void Digitizer::setUseSCDistortions(std::string_view finp)
 
 void Digitizer::setStartTime(double time)
 {
+  // this is setting the first timebin index for the digit container
+  // note that negative times w.r.t start of timeframe/data-taking == mOutputDigitTimeOffset
+  // will yield the 0-th bin (due to casting logic in sampaProcessing)
   SAMPAProcessing& sampaProcessing = SAMPAProcessing::instance();
   sampaProcessing.updateParameters(mVDrift);
-  mDigitContainer.setStartTime(sampaProcessing.getTimeBinFromTime(time - mOutputDigitTimeOffset));
+  const auto timediff = time - mOutputDigitTimeOffset;
+  const auto starttimebin = sampaProcessing.getTimeBinFromTime(timediff);
+  mDigitContainer.setStartTime(starttimebin);
 }
 
 void Digitizer::setLumiScaleFactor()