if set to 0, trig difference do not stop scans, used
              for debugging
TomoCcd : init readout_time to 0 to force at least one calibration
          frelon readout_time do not use shutter_time
TomoScan : use shutter_time if shutter_type is CCD in scan point time
           calculation.
           added watchdog_trig_difference member (default to 2) to
           change watchdog behaviour if needed