Gravar-mail: [μ-2-(4-Hy­droxy­phen­yl)acetato]-κ(3) O:O,O′;κ(3) O,O′:O′-bis­{aqua­(4,4′-bi­pyridine-κN)bis­[2-(4-hy­droxy­phen­yl)acetato-κ(2) O,O′]holmium(III)} mono­hydrate