rename "p" in "pressure" to be consistent with doxygen docu. Commits like that...

rename "p" in "pressure" to be consistent with doxygen docu. Commits like that help to get towards 20 000 rapidly.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@10016 2fb0f335-1f38-0410-981e-8018bf24f1b0