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
If there is no next frame, a null value should be given.