The increment for each step for the specified axis
              in degrees.  In general, this will agree with
              _diffrn_scan_frame_axis.angle_increment . The
              sum of the values of _diffrn_scan_frame_axis.angle  and
              _diffrn_scan_frame_axis.angle_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.angle_increment  will be
              representative
              of the ensemble of values of
              _diffrn_scan_frame_axis.angle_increment  (e.g.
              the mean).