1. Department of Mathematics, Uppsala University, Uppsala 75106, Sweden
2. Structure and Motion Laboratory, The Royal Veterinary College, University of London, Hatfield, Hertfordshire AL9 7TA, UK
3. Department of Computer Science, University College of London, Gower Street, London WC1E 6BT, UK
4. Department of Physiology, Development and Neuroscience, University of Cambridge, Downing Street, Cambridge CB2 3DY, UK
5. Department of Biosciences, College of Science, Swansea University, Swansea SA2 8PP, UK