1. Institute of Science and Technology Austria, 3400 Klosterneuburg, Austria.
2. Department of Computer Science, University of York, Deramore Lane, York YO10 5GH, UK.
3. Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
4. School of Science and Technology, Physics Division, University of Camerino, Camerino (MC), Italy.
5. INFN, Sezione di Perugia, Perugia Italy.
6. CNR-INO, Florence, Italy.