1. Institute of Cross-Disciplinary Physics and Complex Systems, IFISC (UIB-CSIC) 1 , 07122 Palma de Mallorca, Spain
2. Department of Electrical, Electronics and Computer Science Engineering, University of Catania 2 , I-95125 Catania, Italy
3. Istituto di Analisi dei Sistemi ed Informatica “A. Ruberti”, Consiglio Nazionale delle Ricerche (IASI-CNR) 3 , 00185 Roma, Italy