The increment for each step for the specified axis
in millimetres. In general, this will agree with
_diffrn_scan_frame_axis.displacement_rstrt_incr.
The sum of the values of
_diffrn_scan_frame_axis.displacement,
_diffrn_scan_frame_axis.displacement_increment and
_diffrn_scan_frame_axis.displacement_rstrt_incr is the
angular setting of the axis at the start of the integration
time for the next frame relative to a given frame and
should equal _diffrn_scan_frame_axis.displacement
for this next frame. If the individual frame values
vary, then the value of
_diffrn_scan_axis.displacement_rstrt_incr will be
representative of the ensemble of values of
_diffrn_scan_frame_axis.displacement_rstrt_incr (e.g.
the mean).