1. Département d'Informatique, University of Geneva, CH-1211 Genève 4, Switzerland, Département de Physique Théorique, Université de Genève, CH 1211 Genève 4, Switzerland, Institute for Theoretical Physics, Eötvös University, 1088 Budapest, Puskin u. 5-7, Hungary, and Department of Physical Chemistry, Technical University of Budapest, H-1521 Budapest, Hungary