1. Department of Physics and Astronomy, Aarhus University, DK-8000 Aarhus C, Denmark
2. Department of Physics, Lund University, SE-22100 Lund, Sweden
3. Department of Physics, University of York, York YO10 5DD, U.K.
4. University of Jyvaskyla, Department of Physics, P. O. Box 35, FI-40014 University of Jyvaskyla, Finland
5. Department of Nuclear Physics, The Australian National University, ACT 0200, Australia
6. Instituto de Estructura de la Materia, CSIC, Serrano 113 bis, E-28006 Madrid, Spain