box models: move extrusion factor to problem and query it via the volume variables

this has the advantage that the common infrastructure for the box
model does not require to access the spatial parameters of a
problem. (the latter are not always present, e.g. in stokes)

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@6574 2fb0f335-1f38-0410-981e-8018bf24f1b0