Crystal structure of catena-bis(μ-dicyanamido)-bis[2-(4-pyridyl)- 4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide-κN)zinc(II) — 2-(4-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (1:2), Zn(C12H16N3O2)2(C2N3)2 · 2C52H64N18O8