Unequivocal characterisation of a [Ln(terpy)(NO3)3·(H2O)] complex.: The synthesis and structure of [M(terpy)(NO3)3·(H2O)] (M=Eu, Tb); a comparison with the structure of [Eu(bipy)2(NO3)3] and with other europium nitrate complexes {terpy=2,2′:6′,2″-terpyridyl; bipy=2,2′-bipyridyl}