TRD: CalibratorVdExB: Avoid divide by zero

In the TrackletTransformer we query multiple times for the vDrift value
to divide by it.
The fit can now not be zero, minimum 0.01.

I promoted the default value to a constant.

Signed-off-by: Felix Schlepper <felix.schlepper@cern.ch>

Author: f3sch

DataFormats/Detectors/TRD/include/DataFormatsTRD/Constants.h

@@ -64,10 +64,12 @@ constexpr float GRANULARITYTRKLSLOPE = 1.f / PADGRANULARITYTRKLSLOPE; ///< granu
 constexpr int ADCBASELINE = 10;                                       ///< baseline in ADC units
 
 // OS: Should this not be flexible for example in case of Kr calib?
-constexpr int TIMEBINS = 30;           ///< the number of time bins
-constexpr float MAXIMPACTANGLE = 25.f; ///< the maximum impact angle for tracks relative to the TRD detector plane to be considered for vDrift and ExB calibration
-constexpr int NBINSANGLEDIFF = 25;     ///< the number of bins for the track angle used for the vDrift and ExB calibration based on the tracking
-constexpr double ANODEPLANE = 0.0335;  ///< distance of the TRD anode plane from the drift cathodes in [m]
+constexpr int TIMEBINS = 30;            ///< the number of time bins
+constexpr float MAXIMPACTANGLE = 25.f;  ///< the maximum impact angle for tracks relative to the TRD detector plane to be considered for vDrift and ExB calibration
+constexpr int NBINSANGLEDIFF = 25;      ///< the number of bins for the track angle used for the vDrift and ExB calibration based on the tracking
+constexpr double ANODEPLANE = 0.0335;   ///< distance of the TRD anode plane from the drift cathodes in [m]
+constexpr double VDRIFTDEFAULT = 1.546; ///< default value for vDrift
+constexpr double EXBDEFAULT = 0.0;      ///< default value for LorentzAngle
 
 // Trigger parameters
 constexpr double READOUT_TIME = 3000;                  ///< the time the readout takes, as 30 TB = 3 micro-s.

Detectors/TRD/calibration/src/CalibratorVdExB.cxx

@@ -113,7 +113,7 @@ void CalibratorVdExB::initProcessing()
   mFitter.SetFCN<FitFunctor>(2, mFitFunctor, mParamsStart);
   mFitter.Config().ParSettings(ParamIndex::LA).SetLimits(-0.7, 0.7);
   mFitter.Config().ParSettings(ParamIndex::LA).SetStepSize(.01);
-  mFitter.Config().ParSettings(ParamIndex::VD).SetLimits(0., 3.);
+  mFitter.Config().ParSettings(ParamIndex::VD).SetLimits(0.01, 3.);
   mFitter.Config().ParSettings(ParamIndex::VD).SetStepSize(.01);
   ROOT::Math::MinimizerOptions opt;
   opt.SetMinimizerType("Minuit2");