1. Institute of Physics, University of Szczecin, Wielkopolska 15, 70‐451 Szczecin, Poland
2. Institute for Theoretical Physics, University of Wrocław, Max Born Pl. 9, 50‐205 Wrocław, Poland
3. Bogolyubov Laboratory of Theoretical Physics, Joint Institute for Nuclear Research, Joliot‐Curie Street 6, 141980 Dubna, Russia