1. Italian National Council of Research, ISTI-CNR, Pisa, Italy. E-mails: Paolo.Baronti@isti.cnr.it, Paolo.Barsocchi@isti.cnr.it, Antonino.Crivello@isti.cnr.it, Michele.Girolami@isti.cnr.it, Fabio.Mavilia@isti.cnr.it, Filippo.Palumbo@isti.cnr.it
2. Department of Computer Science, University of Pisa, Pisa, Italy. E-mail: stefano.chessa@unipi.it