The Exelixis Lab, Scientific Computing Group, Heidelberg Institute for Theoretical Studies, Schloss-Wolfsbrunnenweg 35, D-69118 Heidelberg, Graduate School for Computing in Medicine and Life Sciences, Institut für Neuro- und Bioinformatik, University of Lübeck, 23538 Lübeck and Karlsruhe Institute of Technology, Institute for Theoretical Informatics, Postfach 6980, 76128 Karlsruhe, Germany.