More than one pressure value can now be fixed inside the domain if a

finite volume pressure model is used. Additionally, the pressure field
is now initialized using the problem.initial() function. Giving
meaningful initializations can
improve the convergence of iterative linear solvers which start the
iterations from the values given in the solution vector

   -reviewed by Bernd



git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@10693 2fb0f335-1f38-0410-981e-8018bf24f1b0