Data items in the PHASING_MIR_DER category record details
               about individual derivatives used in the phasing of the
               structure when methods involving isomorphous replacement are
               involved.
               A derivative in this context does not necessarily equate with
               a data set; for instance, the same data set could be used to
               one resolution limit as an isomorphous scatterer and to a
               different resolution (and with a different sigma cutoff) as an
               anomalous scatterer. These would be treated as two distinct
               derivatives, although both derivatives would point to the same
               data sets via _phasing_MIR_der.der_set_id and
               _phasing_MIR_der.native_set_id.