The value of _diffrn_data_frame.center_slow is
the slow index axis beam center 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 center of the first pixel to
the point at which the Z-axis (which should be colinear with the
beam) intersects the face of the detector, if in fact is does.
At the time of the measurement the current setting of detector
positioner 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.