1. Group of NonLinear Physics, Faculty of Physics, University of Santiago de Compostela, 15782 Santiago de Compostela, Spain
2. Galician Center for Mathematical Research and Technology (CITMAga), 15782 Santiago de Compostela, Spain
3. Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138-1204, USA
4. Santa Fe Institute, Santa Fe, New Mexico 87501, USA