Added PorousMediaBoxProblem in dumux/boxmodels/common. This implements

FS 116. It unifies common functionality of the individual porous media
box models and renders the model-specific base problems like TwoPProblem
obsolete. The model-specific problems have been kept and marked
deprecated. All test problems in test/boxmodels apart from Richards now
directly inherit from PorousMediaBoxProblem.

Reviewed by Christoph.


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@8064 2fb0f335-1f38-0410-981e-8018bf24f1b0