Diamagnetic and Paramagnetic Keggin Polyoxometalate Salts Containing 1-D and 3-D decamethylferrocenium Networks: Preparation, Crystal Structures and Magnetic Properties of [Fe(C5Me5)2]4(POM)(solv)n (POM = [SiMo12O40]4-, [SiW12O40]4-, [PMo12O40]4-, [HFeW12O40]4-; solv = H2O, C3H7ON, CH3CN)