Acknowledgement

We would like to thank N. M. Maclaren, G. J. McMullan and P. R. C. Kent for their assistance. The computational resources for the development of this work were provided by the University of Cambridge High Performance Computing Facility. We acknowledge the support of the U.K. Engineering and Physical Sciences Research Council and Magdalene College, Cambridge (P.D.H.), and of the Natural Sciences and Engineering Research Council of Canada (M.C.).