Changes for the salinity model in the fluidsystem:

- Bugfix: Component Brine now uses the set salinity (for density calulation etc)
- Property Salinity now called ProblemSalinity, which can be set via the parameter tree (Group: Problem, Param: Salinity)

Co2 tests adapted accordingly.

reviewd by & developped with Alex

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@9358 2fb0f335-1f38-0410-981e-8018bf24f1b0