Gravar-mail: Poly[[μ-aqua-bis­(μ-4,4′-bipyridine-κ(2) N:N′)bis­(μ-3-hy­droxy­adamantane-1-carboxyl­ato-κ(2) O:O′)bis­(3-hy­droxy­adamantane-1-carboxyl­ato-κO)dicobalt(II)] hepta­hydrate]