1. Department of Physics and Astronomy, Uppsala University, 751 20 Uppsala, Sweden
2. Center for Free-Electron Laser Science, Department of Physics, University of Hamburg, 22761 Hamburg, Germany
3. Department of Physics, Stockholm University, AlbaNova University Center, 106 91 Stockholm, Sweden
4. Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland
5. Institute of Nuclear Physics, Polish Academy of Sciences, PL-31342 Krakow, Poland
6. European XFEL GmbH, 22869 Schenefeld, Germany