1. Department of Physics, University of Hamburg, Jungiusstrasse 11, D-20355 Hamburg, Germany
2. Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia
3. Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, SI-1000 Ljubljana, Slovenia
4. Department of Theoretical Physics and Applied Mathematics, Ural Federal University, 19 Mira Street, Yekaterinburg 620002, Russia
5. Department of Physics and Astronomy, Uppsala University, P.O. Box 516, Uppsala SE-751 21, Sweden