mCBM: Integration of peak fitting in CheckTiming

• Add FitPeaks() functionality to perform fitting of peaks in time correlations of digis
• Option to call FitPeaks() iteratively after a number of timeslices (fNrTsForFit>=1) during CbmCheckTiming::Exec
• For fNrTsForFit<1 FitPeaks() is only executed at the end of the run in CbmCheckTiming::Finish
• Fitted function is overlaid on corresponding histogram distributions
• fNrTsForFit for iterative fitting can be adjusted in macro checkTiming.C