Magnetic‐Field‐Dependent Electronic Relaxation of Gd3+ in Aqueous Solutions of the Complexes Gd(H2O)83+, Gd(propane‐1,3‐diamine‐N,N,N′,N′‐tetraacetate)(H2O)2−, and Gd(N,N′‐bis[(N‐methylcarbamoyl)methyl‐3‐azapentane‐1,5‐diamine‐3,N,N′‐triacetate)(H2O)] of interest in magnetic‐resonance imaging