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.