1 Mathematics Institute, University of Warwick, Coventry, UK; Zeeman Institute for Systems Biology and Infectious Disease Epidemiology Research, University of Warwick, Coventry, UK. Electronic address: robin.n.thompson@warwick.ac.uk.
2 Mathematics Institute, University of Warwick, Coventry, UK; Zeeman Institute for Systems Biology and Infectious Disease Epidemiology Research, University of Warwick, Coventry, UK; School of Life Sciences, University of Warwick, Coventry, UK.
3 Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge, UK.