box models: reorganization for context objects

allows to better decouple the problem from the model and will also
make the generic implementation of multiple spatial and temporal
discretizations _much_ easier.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@6671 2fb0f335-1f38-0410-981e-8018bf24f1b0