1. School of Mathematics and Statistics, University of Sheffield, Sheffield S3 7RH, UK
2. Biosciences, College of Life and Environmental Sciences, University of Exeter Cornwall Campus, Penryn TR10 9EZ, UK
3. MRC Epidemiology Unit, Institute of Metabolic Science, Addenbrooke's Hospital, Cambridge CB2 0QQ, UK
4. Department of Mathematics and the Maxwell Institute for Mathematical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, UK