German National Research Center for Information Technology (GMD), Institute for Algorithms and Scientific Computing (SCAI), Schloss Birlinghoven, 53754 Sankt Augustin, Germany. Rarey@gmd.de|German National Research Center for Information Technology (GMD), Institute for Algorithms and Scientific Computing (SCAI), Schloss Birlinghoven, 53754 Sankt Augustin, Germany. Rarey@gmd.de|German National Research Center for Information Technology (GMD), Institute for Algorithms and Scientific Computing (SCAI), Schloss Birlinghoven, 53754 Sankt Augustin, Germany. Rarey@gmd.de