[1] Univ Italian Switzerland, Inst Computat Sci, CH-6900 Lugano, Switzerland
[2] Univ Insubria, Dept Sci & High Technol, I-22100 Como, Italy
[3] Max Planck Inst Plasma Phys, Div Numer Methods Plasma Phys, D-85748 Garching, Germany
[4] Uppsala Univ, Dept Informat Technol, POB 337, SE-75105 Uppsala, Sweden