[Stokes, Stokes2c]

Move (deprecate & add) molarDensity from Stokes to Stokes2c where it
belongs.
After the release, despite removing the deprecated code the member in
StokesFluxVariables must be un-commented and the "this->" must be
removed, too.
(reviewed by klaus)


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@9594 2fb0f335-1f38-0410-981e-8018bf24f1b0