values are integer numerals, optionally followed by an insertion code of the form ^X where X is a letter. in regex terms: '[0-9+](^[A-Z])?relates a residue in a PDB structure to the coordinate given
to it in its ATOM records in the traditional PDB formate.g. the 1st ATOM record for the C chain of 1JWS starts 'ATOM 3015 N PRO C 306 ...'; hence pdb_coord '306'