box local jacobian: modify the base epsilon to 10^-10

and make sure that the base epsilon is still at least 4 orders of
magnitude larger than the machine precision...

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@7583 2fb0f335-1f38-0410-981e-8018bf24f1b0