The metal ammine cyanide aromatic clathrates. XIII. The crystal structure of the Hofmann-type biphenyl clathrate, diamminenickel(II) tetracyanonickelate(II) di-biphenyl: Ni(NH<sub>3</sub>)<sub>2</sub>Ni(CN)<sub>4</sub>.2C<sub>12</sub>H<sub>10</sub>