The value of _diffrn_data_frame.center_slow is
the slow index axis beam centre position relative to the detector
element face in the units specified in the data item
_diffrn_data_frame.center_units along the slow
axis of the detector from the centre of the first pixel to
the point at which the Z axis (which should be collinear with the
beam) intersects the face of the detector, if in fact it does.
At the time of the measurement the current settings of
the detector positioner for the given frame are used.
It is important to note that the sense of the axis is used,
rather than the sign of the pixel-to-pixel increments.