Acknowledgements

Work at Los Alamos (NM, USA) is supported by the US DOE Basic Sciences for the National Nuclear Security Administration of the US Department of Energy under Contract No. DE-AC52-06NA25396. Work on the project at Nordita (Stockholm, Sweden) is supported by the Swedish Research Council Grant No. 638-2013-9243, the Knut and Alice Wallenberg Foundation, and the European Research Council under the European Union’s Seventh Framework Program (FP/2207-2013)/ERC Grant Agreement No. DM-321031.