1. Department of Applied Physics, Aalto University School of Science, 00076 Aalto, Finland
2. Jožef Stefan Institute, Jamova 39, SI-1001 Ljubljana, Slovenia
3. Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, SI-1000 Ljubljana, Slovenia
4. Computational Physics Laboratory, Physics Unit, Faculty of Engineering and Natural Sciences, Tampere University, P.O. Box 692, FI-33014 Tampere, Finland