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.