The setting of the specified axis in millimetres
              against which measurements of the reference beam centre
              and reference detector distance should be made.
              In general, this will agree with
              _diffrn_scan_frame_axis.reference_displacement.
              If the individual frame values vary, then the value of
              _diffrn_scan_axis.reference_displacement  will be
              representative of the ensemble of values of
              _diffrn_scan_frame_axis.reference_displacement  (e.g.
              the mean).