1Institut d’Organització i Control i Departament de Matemàtiques, Universitat Politècnica de Catalunya, EPSEVG, Avinguda Víctor Balaguer 1, Vilanova i la Geltrú 08800, Spain
2Department of Physics and Astronomy, University of Florence and INFN, 50019 Sesto Fiorentino (FI), Italy
3Maxwell Institute and School of Mathematics, The University of Edinburgh, James Clerk Maxwell Building, Peter Guthrie Tait Road, King’s Buildings, Edinburgh EH9 3FD, United Kingdom