1. Inria, 75589 Paris, France, and CERMICS, Ecole des Ponts, 77455 Marne-la-Vallée, France.
2. Institute of Mathematics, University of Augsburg, 86159 Augsburg, Germany.
3. Institute for Applied and Numerical Mathematics, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany.