1. Center for Free-Electron Laser Science, Deutsches Elektronen-Synchrotron DESY, Notkestraße 85, 22607 Hamburg, Germany
2. Department of Physics, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany
3. Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom
4. Center for Ultrafast Imaging, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany