1. Department of Mathematics and Natural Science; University of Stavanger; N-4036 Stavanger Norway
2. Department of Chemistry; Norwegian University of Science and Technology; N-7491 Trondheim Norway
3. Laboratoire de Cristallographie; École Polytechnique Fédérale de Lausanne; BSP, Lausanne, CH-1015 Lausanne, Switzerland. Swiss-Norwegian beamlines, ESRF, BP220, Grenoble, Cedex 9 France