1. Centre for Mathematical Biology, Mathematical Institute, University of Oxford, OX1 3LB Oxford, UK
2. Department of Engineering Sciences and Methods, University of Modena and Reggio Emilia, I-42100 Reggio Emilia, Italy
3. Department of Mathematical Sciences, Polytechnic University of Marche, I-60131 Ancona, Italy
4. Department of Electronic and Telecommunications, University of Florence, I-50139 Florence, Italy