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).