1. Department of Mathematical Physics and Astronomy, Gent University, 9000 Gent, Belgium;
2. Department of Physiology, Academic Medical Center, University of Amsterdam, 1105 AZ Amsterdam; and Department of Medical Physiology, University Medical Center Utrecht, 3584 CG Utrecht;
3. Institute for Theoretical Biology, Humboldt University, 10115 Berlin, Germany; and
4. Department of Theoretical Biology, Utrecht University, 3584 CG Utrecht, The Netherlands