1. Fachbereich Medien, Fachhochschule Düsseldorf, University of Applied Sciences, Josef-Gockeln-Str. 9, 40474 Düsseldorf, Germany
2. Institute for Theoretical Physics, University of Cologne, 50923 Köln, Germany
3. Department of Physics and Astronomy, University of Missouri-Columbia, Columbia, MO 65211, USA