box models: add a few additional valgrind checks, allow problems to cap time step sizes more easily

to cap the time step size a problem can overload the maxTimeStepSize() method.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@5461 2fb0f335-1f38-0410-981e-8018bf24f1b0