1. Corresponding author. Institute of Industrial Mathematics, Johannes Kepler University Linz, Altenbergerstraße 69, A-4040 Linz, Austria.
2. Johann Radon Institute Linz, Altenbergerstraße 69, A-4040 Linz, Austria.
3. Department of Astrophysics, University of Vienna, Türkenschanzstraße 17, A-1180 Vienna, Austria.
4. School of Mathematics, Trinity College, Dublin 2, Ireland.
5. Johannes Kepler University Linz, and Johann Radon Institute Linz, Altenbergerstraße 69, A-4040 Linz, Austria.