The increment for this frame for the displacement setting of
the specified axis in millimetres. 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 and should equal
_diffrn_scan_frame_axis.displacement for this next frame.