1. Laboratory of Applied Mathematics; Department of Civil, Environmental and Mechanical Engineering, University of Trento, via Mesiano 77; Trento I-38123 Italy
2. Department of Science and High Technology; University of Insubria, via Valleggio 11; Como I-22100 Italy
3. Max-Planck-Institut für Plasmaphysik Boltzmannstraße 2, D-85748 Garching; Germany
4. Department of Information Technology; Division of Scientific Computing, Uppsala University; Uppsala Box 337 SE-751 05 Sweden