1. Department of Mathematics and Statistics, University of Exeter, Exeter EX4 4QL, UK.
2. Sport & Health Sciences, St Luke’s Campus, Exeter EX1 2LU UK.
3. Department of Mathematics and Statistics, University of Exeter, Exeter EX4 4QL, UK, Living Systems Institute, EPSRC Hub for Quantitative Modelling in Healthcare, University of Exeter, Stocker Road, Exeter EX4 4QD, UK, and Institute for Advanced Study, Technical University of Munich, D-85748 Garching, Germany.