added posibility to set a fixed pressure at any position in the domain

   - feature can be used if only flux boundary conditions are set
   - further the output functions are changed due to changes in the
     VTKMultiWriter



git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@6140 2fb0f335-1f38-0410-981e-8018bf24f1b0