Structural characterization of the {3[HBPMTU](+)center dot 3[X](-)center dot nH(2)O} salts (BPMTU=1,3-bis(3-pyridylmethyl)-2-thiourea and X = Cl, Br, I). A polychlorine network based on O center dot center dot center dot Cl- and OH center dot center dot center dot Cl- interactions