Dramatic Modifications of Magnetic Properties through Dehydration−Rehydration Processes of the Molecular Magnetic Sponges CoCu(obbz)(H2O)4·2H2O and CoCu(obze)(H2O)4·2H2O, with obbz = N,N‘-Bis(2-carboxyphenyl)oxamido and obze = N-(2-Carboxyphenyl)-N‘-(carboxymethyl)oxamido