box: fix for dirichlet boundaries

as described in my e-mail from wednesday. Everything in the stable
repository compiles.

also, some compile fixes for the Mp-Nc model where made to some fluid
systems and the 1p2c index structure got an offset template argument
like the other models.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@4446 2fb0f335-1f38-0410-981e-8018bf24f1b0