The code identifying each value of
               _diffrn_radiation_wavelength.wavelength.
               Items in the DIFFRN_RADIATION_WAVELENGTH category are looped
               when multiple wavelengths are used.
               This code is used to link with the DIFFRN_REFLN category.
               The _diffrn_refln.wavelength_id codes must match one of
               the codes defined in this category.