1. VTT Technical Research Centre of Finland, P.O. Box 1100, 90571 Oulu, Finland
2. VTT Technical Research Centre of Finland, P.O. Box 1000, 02044 VTT, Finland
3. Department of Statistics, Rice University, MS 138, 6100 Main Street Houston, TX 77251-1892, USA
4. Ecole d'ingénieurs et d'architectes de Fribourg, Bd de Pérolles 80 - CP 32, CH-1705 Fribourg, Switzerland