1. Department for Computing Science, University of Oldenburg, Oldenburg D-26129, Germany and Institut d'Electronique, de Microelectronique et de Nanotechnologie, CNRS UMR 8520/University of Lille 1, Villeneuve d'Ascq F-59652, France
2. Department for Computing Science, University of Oldenburg, Oldenburg D-26129, Germany
3. Institut d'Electronique, de Microelectronique et de Nanotechnologie, CNRS UMR 8520, Villeneuve d'Ascq F-59652, France