This replaces the old mapping member by constructing it from self.chain_mapping. This function returns a mapping from
        chain IDs in pdb_name1 to chain IDs in pdb_name2.

def get_chain_mapping(self, pdb_name1, pdb_name2):
        '''This replaces the old mapping member by constructing it from self.chain_mapping. This function returns a mapping from
        chain IDs in pdb_name1 to chain IDs in pdb_name2.'''
        raise Exception('Implement. Map pdb_namex to its equivalence class, call get_representative_chain_mapping, and something something.')
        pprint.pprint(self.chain_mapping)

        d = {}
        for pdb1_chain_id, matched_chain_list in self.chain_mapping[(pdb_name1, pdb_name2)].iteritems():
            d[pdb1_chain_id] = matched_chain_list.get_related_chains_ids(pdb_name2)
        return d