Avoid ihm_model_id in atom_site table

[tomgoddard]

Currently IHM inserts a new field ihm_model_id into the atom_site table. This complicates code that reads these files because it will likely use an existing mmCIF reader for atom_site that does not know about the ihm_model_id. It might be better to use the atom_site pdbx_PDB_model_num field. That field is necessary in any case to correctly parse the atom_site table into multiple models. The mapping of pdbx_PDB_model_num to ihm_model_id could be another IHM table allowing the IHM extension to be self-contained without injecting fields into categories from other dictionaries.