[1] Univ Zaragoza, Inst Biocomputat & Phys Complex Syst BIFI, E-50009 Zaragoza, Spain
[2] Univ Rome Roma Tre, Dept Informat & Automat, I-00146 Rome, Italy
[3] Univ Zaragoza, Dept Condensed Matter Phys, E-50009 Zaragoza, Spain
[4] Univ London, Sch Math Sci, London E1 4NS, England
[5] Univ Catania, Dipartimento Fis & Astron, Catania, Italy
[6] Ist Nazl Fis Nucl, I-95123 Catania, Italy
[7] Scuola Super Catania, Lab Sistemi Complessi, I-95123 Catania, Italy
[8] Univ Zaragoza, Dept Theoret Phys, E-50009 Zaragoza, Spain