1. Department of Engineering, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ, U.K. sss40@cam.ac.uk
2. Department of Information Technology, Uppsala University, Lägerhyddsv. 2, Uppsala 751 05, Sweden fredrik.lindsten@it.uu.se
3. Centre de Mathématiques Appliquées, École Polytechnique, Route de Saclay, 91128 Palaiseau Cedex, France eric.moulines@polytechnique.edu