1. Department of Physics, Njord Centre, University of Oslo, P.O. Box 1048, 0316 Oslo, Norway
2. École Normale Supérieure, PSL Research University, 45 rue d'Ulm, 75005 Paris, France
3. Institute of Science and Technology Austria, Am Campus 1, A-3400 Klosterneuburg, Austria
4. Niels Bohr Institute, University of Copenhagen, Blegdamsvej 17, Copenhagen, Denmark