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).