Fixed bugs:
- was reading HETATM lines as well as ATOM in atom_site
- auth_seq_num with '?' not taken now when populating the pdbresser2resser map (same behaviour as in PdbasePdb)
- now using chainCodeStr and auth_asym_id to identify chains in pdbx_poly_seq_scheme, struct_conf and struct_sheet_range. atom_site is not guaranteed to appear in file before all the others so we can't rely on having read a chainCode (asym_id) when parsing the other elements