1. Mullard Space Science Laboratory, University College London (UCL), Dorking RH5 6NT, UK
2. Alan Turing Institute, London NW1 2DB, UK
3. Department of Computer Science, University College London (UCL), London WC1E 6BT, UK
4. School of Electronics and Computer Science, University of Southampton, Southampton SO17 1BJ, UK
5. School of Mathematical and Computer Sciences, Heriot-Watt University, Edinburgh EH14 4AS, UK