1. Corresponding author. Johann Radon Institute Linz, Altenbergerstraße 69, 4040 Linz, Austria.
2. Faculty of Mathematics, University of Vienna, Oskar Morgenstern-Platz 1, 1090 Vienna, Austria.
3. Institute of Industrial Mathematics, Johannes Kepler University Linz, Altenbergerstraße 69, 4040 Linz, Austria, and Johann Radon Institute Linz, Altenbergerstraße 69, 4040 Linz, Austria.
4. Center for Medical Physics and Biomedical Engineering, Medical University of Vienna, Währinger Gürtel 18–20, 1090 Vienna, Austria.