Change the calculation of the weighting factor in the tpfa adaptive velocity...

Change the calculation of the weighting factor in the tpfa adaptive velocity calculation, so that it also works with UGGrid. Discussed with Markus.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@11446 2fb0f335-1f38-0410-981e-8018bf24f1b0