Implemented general flux variables according to FS #159. Adapted

(simplified) box 2p model and local residual correspondingly. The
original 2p flux variables are obsolete (once also the 2pni model has
been adapted) and have been marked deprecated. 

Reviewed by Philipp.


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@8692 2fb0f335-1f38-0410-981e-8018bf24f1b0