changed the fluidsystem in the forchheimer 1p test problem: us...

changed the fluidsystem in the forchheimer 1p test problem: us h2oairfluidsystem, in order to be able to specify water with constant properties. 

This way the forchheimer velocity can be analytically checked. 

-> updated reference solution

removed some (debugging???) code from the h2oairfluidsystem: For the case of incompressible water, the pressure was messed up. 

reviewed by Bernd

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@9040 2fb0f335-1f38-0410-981e-8018bf24f1b0