Oxalato and squarato ligands in nickel(II) complexes of tetraazacycloalkanes. Solution and solid-state studies. Crystal and molecular structures of (.mu.-oxalato)bis[(1,7-dimethyl-1,4,7,10-tetraazacyclododecane)nickel(II)] perchlorate dihydrate and of bis[diaquo(1,4,7,10-tetraazacyclododecane)nickel(II)] squarate diperchlorate