1 IBM Research Europe - Zurich, Rüschlikon, Switzerland; École Polytechnique Fédérale de Lausanne, School of Life Sciences and School of Computer and Communication Sciences, Lausanne EPFL, Switzerland. Electronic address: ans@zurich.ibm.com.
2 IBM Research Europe - Zurich, Rüschlikon, Switzerland.
3 École Polytechnique Fédérale de Lausanne, School of Life Sciences and School of Computer and Communication Sciences, Lausanne EPFL, Switzerland.