1. National Research Council of Italy IEIIT-CNR, c/o Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy.
2. Department of Mathematical Sciences “Giuseppe Luigi Lagrange,” Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy.
3. Computer Science Department, University of Turin, Via Verdi 8, 10124 Torino, Italy.
4. Department of Electronics and Telecommunications DET, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy.