Unequivocal characterisation of a [Ln(terpy)(NO3)(3) center dot (H2O)] complex. The synthesis and structure of [M(terpy)(NO3)(3) center dot (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}