The value of _diffrn_detector.number_of_axes gives the
number of axes of the positioner for the detector identified
by _diffrn_detector.id.
The word 'positioner' is a general term used in
instrumentation design for devices that are used to change
the positions of portions of apparatus by linear
translation, rotation or combinations of such motions.
Axes which are used to provide a coordinate system for the
face of an area detector should not be counted for this
data item.
The description of each axis should be provided by entries
in DIFFRN_DETECTOR_AXIS.