Magnetic-Field-Dependent Electronic Relaxation of Gd^3^+ in Aqueous Solutions of the Complexes [Gd(H~2O)~8]^3^-, [Gd(propane-1,3-diamine-N,N,N',N'-tetraacetate)(H~2O)~2]^-, and [Gd(N,N'-bis[(N-methylcarbamoyl)methyl]-3-azapentane-1,5-diamine-3,N,N'-triacetate)(H~2O)] of Interest in Magnetic-Resonance Imaging