1. Biomimetics and Intelligent Systems Group , University of Oulu , P.O. BOX 8000 , Oulu 90014 , Finland
2. Research Unit of Mathematical Sciences , University of Oulu , P.O. BOX 3000 , Oulu 90014 , Finland
3. Research Unit of Mathematical Sciences , University of Oulu , P.O. BOX 3000 , Oulu 90014 , Finland ; and Moscow Centre of Fundamental and Applied Mathematics – Lomonosov Moscow State University, Moscow, Russia