1. Department of Mathematics and Geosciences, University of Trieste, 34128 Trieste, Italy
2. Institute of Physics, Faculty of Philosophy and Science, Silesian University in Opava, Bezručovo Náměstí 13, 74601 Opava, Czech Republic
3. Institute of Physics and Research Center for Theoretical Physics and Astrophysics, Faculty of Philosophy and Science, Silesian University in Opava, Bezručovo Náměstí 13, 74601 Opava, Czech Republic