Magnetic Excitations in Polyoxometalate Clusters Observed by Inelastic Neutron Scattering: Evidence for Anisotropic Ferromagnetic Exchange Interactions in the Tetrameric Cobalt(II) Cluster [Co₄(H₂O)₂(PW₉O₃₄)₂]¹⁰⁻. Comparison with the Magnetic and Specific Heat Properties