Approximate average time in seconds between the end
              of integration of each step of the scan and the start
              of integration of the next step.
              In general, this will agree with
              _diffrn_scan_frame.time_rstrt_incr .  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.   If the individual frame values vary, then 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).