Institute for Algorithms and Scientific Computing (SCAI), German National Research Center for Information Technology (GMD), 53754 Sankt Augustin, Germany.|Institute for Algorithms and Scientific Computing (SCAI), German National Research Center for Information Technology (GMD), 53754 Sankt Augustin, Germany.|Institute for Algorithms and Scientific Computing (SCAI), German National Research Center for Information Technology (GMD), 53754 Sankt Augustin, Germany.