The time in seconds between the end of integration of this step of the scan
              and the start of integration of the next step.
              The sum of the values of _diffrn_scan_frame.integration_time
              and  _diffrn_scan_frame.time_rstrt_incr  is the
              time from the start of integration of one frame and the start of
              integration for the next frame and should equal the value of
              _diffrn_scan_frame.time_period  for this
              frame.   The value of _diffrn_scan.time_rstrt_incr  will be
              representative of the ensemble of values of
              _diffrn_scan_frame.time_rstrt_incr  (e.g.
              the mean).
              If there is no next frame, a null value should be given.