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).