Centre for Mathematical Biology, Mathematical Institute, University of Oxford, Oxford, United Kingdom. m.tindall@reading.ac.uk|Centre for Mathematical Biology, Mathematical Institute, University of Oxford, Oxford, United Kingdom. m.tindall@reading.ac.uk|Centre for Mathematical Biology, Mathematical Institute, University of Oxford, Oxford, United Kingdom. m.tindall@reading.ac.uk|Centre for Mathematical Biology, Mathematical Institute, University of Oxford, Oxford, United Kingdom. m.tindall@reading.ac.uk