added the zeroeq turbulence models from dumux-devel
added zeroeq test problems
to be done:
 - general docu and doxygen docu
 - model description for handbook
 - dune compatibilites 2.3 and >2.3

reviewed by gruenich


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@14861 2fb0f335-1f38-0410-981e-8018bf24f1b0