Fix the sTGC place holder time-to-tdo calibration

The "place holder" calibration shifts digit time by a value, which is set to 100 ns, so that time becomes positive in order to avoid FPE warnings when writing RDOs to the disk, since the time variable in the persistent RDO class is an unsigned integer. The value of the shift has to correspond to the smallest time considered for digitization, which is defined at Line12 in sTGC_Digitization/python/sTGC_DigitizationConfig.py. This "place holder" calibration will be replaced by a proper calibration interface that is currently under development.