Polycatenane systems from Co(II) and trans-1,2-bis(4-pyridyl)ethene (bpe). Synthesis and structure of Co(bpe)2(NCS)2·CH3OH, [Co(bpe)2(H2O)2](ClO4)2·2CH3OH and [Co(bpe)2(H2O)2(CH3OH)2](ClO4)2·bpe·H2O