Now, the box 2p model can use the parameter tree for enabling

gravity. In order to not run into serious performance penalties, 
the parameter tree is not parsed every time a FluxVariables object is
constructed. Instead, the 2p problem has been equipped with a function 
const bool enableGravity() const 
which is called from the flux variables. 

An example for the usage is provided in 
dumux-devel/test/boxmodels/2pwithparam.


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@6395 2fb0f335-1f38-0410-981e-8018bf24f1b0