Poly[[(μ 2 -benzene-1,4-dicarboxylato-κ 4O 1 , O 1′ : O 4 , O 4′ )(μ 2 -di-4-pyridyldiazene-κ 2N 1 : N 1′ )cobalt(II)] N , N -dimethylformamide disolvate hemihydrate]