Low-temperature structural and magnetic characterization of the chromium(III) dimers bis(.mu.-hydroxo)bis[tetraamminechromium(III)] dithionate tetrahydrate and bis(.mu.-hydroxo)bis[tetraamminechromium(III)] chloride tetrahydrate, [Cr(NH3)4OH]2(S2O6)2.4H2O and [Cr(NH3)4OH]2Cl4.4H2O