1. Department of Theoretical Physics, Faculty of Physics, Sofia University, Sofia 1164, Bulgaria
2. School of Mathematical Sciences and STAG Research Centre, University of Southampton, Southampton, SO17 1BJ, United Kingdom
3. Theoretical Astrophysics, Eberhard Karls University of Tübingen, Tübingen 72076, Germany
4. Institute of Mathematics and Informatics, Bulgarian Academy of Sciences, Acad. G. Bonchev St. 8, Sofia 1113, Bulgaria