1. Department of Mathematical Sciences and Interdisciplinary Centre for Mathematical Modelling, Loughborough University, Loughborough LE11 3TU, UK
2. School of Mathematics and the Maxwell Institute for Mathematical Sciences, University of Edinburgh, Edinburgh EH9 3FD, UK
3. Department of Physics and Mathematics, Nottingham Trent University, Clifton Lane, Nottingham NG11 8NS, UK
4. School of Physics and Astronomy, University of Edinburgh, Edinburgh EH9 3FD, UK