support pdbx_struct_mod_residue in mmCIF files

[Greg Couch]

Originally, the equivalent of the PDB file's MODRES record was the chem_comp.mon_nstd_parent_comp_id item. That only works if the component is in the PDB's CCD repository (ligand-expo.rcsb.org). At some point, the PDB added the pdbx_struct_mod_residue table. That should be supported too. Since the pdbx_struct_mod_residue tables is present in mmCIF files from the PDB, that would allow modified residues to be correctly recognized when the web site with the CCD templates is down.

[Greg Couch]

Using the pdbx_struct_mod_residues to find modified residues is not needed because the CCD entries have the single letter code needed. And that is much more reliable. Would rather that the PDB included the "comp_chem.one_letter_code" in its mmCIF files.