map each residue to the corresponding molecule.

def _set_residue_map(self):
        """
        map each residue to the corresponding molecule.
        """
        self.map_residue_to_mol = {}
        lookup = {}
        for idx, mol in enumerate(self.mols):
            if not mol.formula in lookup:
                mol.translate_sites(indices=range(len(mol)),
                                    vector=-mol.center_of_mass)
                lookup[mol.formula] = mol.copy()
            self.map_residue_to_mol["ml{}".format(idx + 1)] = lookup[mol.formula]