Synthesis, properties and crystal structure of a Zn,Nb compound with homo- and heterodinuclear oxalate-bridged units – [{Zn(bpy)2}2(μ-C2O4)][Zn(bpy)2(μ-C2O4)NbO(C2O4)2]2·0.5bpy·7H2O: The impact of weak interactions on the crystal packing