Bring the joy of parallelism also to the common decoupled stuff. Fix the...

Bring the joy of parallelism also to the common decoupled stuff. Fix the pressure matrix assembly in parallel if faces are visited only once. Can be tested with test/decoupled/2p/test_parallel in dumux-devel. Reviewed by Markus.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@9434 2fb0f335-1f38-0410-981e-8018bf24f1b0