The general class of goniometer or device used to support
              and orient the specimen.
              If the value of _diffrn_measurement.device is not given,
              it is implicitly equal to the value of
              _diffrn_measurement.diffrn_id.
              Either _diffrn_measurement.device or
              _diffrn_measurement.id may be used to link to other
              categories.  If the experimental setup admits multiple
              devices, then _diffrn_measurement.id is used to provide
              a unique link.