1. Department of Mathematics, Politecnico Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy
2. Centre for Mathematical Biology, Mathematical Institute, 24-29, St-Giles', Oxford, OX1 3LB, UK
3. Oxford Centre for Integrative Systems Biology, Department of Biochemistry, South Parks Road, Oxford, OX1 3QU, UK