Architectural change todo -- PDBProp objects with the same ID for a GEM-PRO should point to the same object rather than being created under different proteins

[nmih]

This also will make the transition to using the hadoop sequence file for structures much easier.

[nmih]

Will need to move mapped_chains attribute from the StructProp objects to Protein objects, as a dictionary named "structure_to_chain_mapping" and storing {structprop_id: [mapped_chains]}