partial reassemble: Smarter way to calculate the reassemble tolerance

divide the relative tolerance by 10 to get the minimum reassemble
error. for the next newton iteration, reassemble all vertices which
exhibit a relative error which is larger than the geometric mean of
the minimum reassemble tolerance and the relative error of the last
iteration.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@4825 2fb0f335-1f38-0410-981e-8018bf24f1b0