1. Max–Planck–Institut für Intelligente Systeme, Heisenbergstr. 3, 70569 Stuttgart, Germany and Institut für Theoretische Physik IV, Universität Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany
2. Institute of Theoretical Physics, Faculty of Physics, University of Warsaw, ul. Pasteura 5, 02–093 Warszawa, Poland