Increasing the Nuclearity of Magnetic Polyoxometalates. Syntheses, Structures, and Magnetic Properties of Salts of the Heteropoly Complexes [Ni3(H2O)3(PW10O39)H2O]7-, [Ni4(H2O)2(PW9O34)2]10-, and [Ni9(OH)3(H2O)6(HPO4)2(PW9O34)3]16-