Gravar-mail: Structure of poly[di­aqua­[μ-1,2-bis­(pyri­din-4-yl)ethane-κ(2) N:N′]bis­(μ(3)-cyclo­butane-1,1-di­carboxyl­ato-κ(3) O,O′:O′′:O′′′)dimanganese(II)]