- allow coarsening only if all leaf cells of a father element (new
     coarser element) are marked for coarsening
   - additionally: added refine and coarsen thresholds as parameter-properties



git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@8028 2fb0f335-1f38-0410-981e-8018bf24f1b0