1. Department of Mathematics and Statistics, University of Konstanz, D-78457 Konstanz, Germany.
2. Johann Radon Institute for Computational and Applied Mathematics (RICAM), Austrian Academy of Sciences, A-4040 Linz, Austria.
3. Institute for Mathematics and Scientific Computing, University of Graz, 8010 Graz, Austria, and Johann Radon Institute for Computational and Applied Mathematics (RICAM), Austrian Academy of Sciences, A-4040 Linz, Austria.