1. Department of Mathematical Sciences, University of Bath , Bath BA2 7JU , UK
2. Royal United Hospital , Bath BA1 3NG , UK
3. Department of Life Sciences, University of Bath , Bath BA2 7JU , UK
4. School of Engineering and Physical Sciences, School of Mathematical and Computer Sciences, Heriot-Watt University , Edinburgh EH14 4AS , UK
5. Maxwell Institute for Mathematical Sciences , Edinburgh EH8 9BT , UK