Completeness (as a percentage) of symmetry-unique
anomalous difference data within the spherical shell
defined by its diffraction limits
(_reflns_shell.d_resolution_high and
_reflns_shell.d_resolution_low) relative to all
possible symmetry-unique anomalous difference data
within that shell.
In the absence of an anisotropy description this is
identical to _reflns.pdbx_percent_possible_anomalous.