1. Institut de Physique Théorique (FYMA), Université Catholique de Louvain, B-1348 Louvain-la-Neuve, Belgium
2. University of Neuchâtel, Institute of Microtechnology, Pattern Recognition Lab., Rue A.-L. Breguet 2, CH-2000 Neuchâtel, Switzerland
3. Signal Processing Institute, Swiss Federal Institute of Technology, EPFL, Station 11, CH-1015 Lausanne, Switzerland