1. School of Mathematical Sciences and Department of Electrical and Electronic Engineering, University of Nottingham, University Park, NG72RD Nottingham, United Kingdom
2. Numerical Algorithms Group Ltd, Portland Street 53, M1-3LD Manchester, United Kingdom
3. Department of Physics and Astronomy, Alma Mater Studiorum, University of Bologna, Viale Berti Pichat 6/2, 40-127 Bologna, Italy and INDAM Gruppo Nazionale di Fisica Matematica, Piazzale Aldo Moro, 5, 00185 Roma, Italy