[1] Univ Cambridge, Dept Appl Math & Theoret Phys, Cambridge CB3 0WA, England
[2] Univ Paris 06, CNRS, UMR 7607, Modelisat Mecan Lab, F-75252 Paris, France
[3] Univ Aix Marseille 1, IRPHE, CNRS, F-13384 Marseille, France
[4] Univ Aix Marseille 2, IRPHE, CNRS, F-13384 Marseille, France