Email (record): catena-Poly[[tetra­aqua­nickel(II)]-μ-(9,10-dioxo-9,10-dihydro­anthracene-1,4,5,8-tetra­carboxyl­ato)-κ(2) O (1):O (8)-[tetra­aqua­nickel(II)]-μ-4,4′-bipyridine-κ(2) N:N′]