Synthesis, Structure, and Magnetic Properties of (n-Bu<sub>4</sub>N)<sub>2</sub>[{Ni(MeOH)<sub>2</sub>}<sub>2</sub>{Mo(NO)}<sub>2</sub>(μ<sub>3</sub>-OH)<sub>2</sub>(μ-OMe)<sub>4</sub>{Mo<sub>5</sub>O<sub>13</sub>(OMe)<sub>4</sub>(NO)}<sub>2</sub>], a New Type of Polyoxometalate Incorporating a Rhomb-like Cluster