Gravar-mail: catena-Poly[4,4′-bipyridinium [[tetra­aqua­(pyridine-2,6-dicarboxyl­ato-κ(3) O (2),N,O (6))cerate(III)]-μ-pyridine-2,6-dicarboxyl­ato-κ(4) O (2):O (2′),N,O (6)-[(pyridine-2,6-dicarboxyl­ato-κ(3) O (2),N,O (6))cerate(III)]-μ-pyridine-2,6-dicarboxyl­ato-κ(4) O (2),N,O (6):O (6′)] penta­hydrate]