1. LIONS − CEA Saclay 9119, Gif-Sur-Yvette, France, Biophysics, Department of Natural Sciences, The Royal Veterinary and Agricultural University, Thorvaldensevej 40, DK-1871 Frederiksberg C, Denmark, and Institut Laue-Langevin, DS/LSS, 6 rue Jules Horowitz, B.P. 156, 38042 Grenoble, France