Gravar-mail: Bis(μ-3-nitro­benzene-1,2-dicarboxyl­ato)-κ(4) O (1),O (2):O (1),O (1′);κ(4) O (1),O (1′):O (1),O (2)-bis­[triaqua­(6-carb­oxy-2-nitro­benzoato-κ(2) O (1),O (6))neodymium(III)] dihydrate