Metallosupramolecular assembly of dinuclear double helicates incorporating a biphenyl-3,3′-diyl spacer; molecular structure of bis{3,3′-bis[4-(methylsulfanyl)-2,2′-bipyridin-6-yl]biphenyl-χ2N,N′ : χ2N″,N‴]}dicopper(I) hexafluorophosphate