1. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109-8099, USA
2. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109-8099, USA and Department of Physics and Astronomy, University of California, Los Angeles, CA 90095-1547, USA
3. Department of Computer Science, Rensselaer Polytechnic Institute, Troy, NY 12180-3590, USA
4. Computer Science, FEIT, Australian National University, Canberra, ACT 0200, Australia