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