Centre for Mathematical and Computational Biology, Rothamsted Research, Harpenden AL5 2JQ, UK.|Centre for Mathematical and Computational Biology, Rothamsted Research, Harpenden AL5 2JQ, UK.|Centre for Mathematical and Computational Biology, Rothamsted Research, Harpenden AL5 2JQ, UK.|Centre for Mathematical and Computational Biology, Rothamsted Research, Harpenden AL5 2JQ, UK.|Centre for Mathematical and Computational Biology, Rothamsted Research, Harpenden AL5 2JQ, UK.|Centre for Mathematical and Computational Biology, Rothamsted Research, Harpenden AL5 2JQ, UK.