remove compiler warnings due to changed access to transported Quantity

little docu improvements
cleanup of fvpressurecompositional, first steps towards general method for volume derivatives
cleanup of vector multiplications (concerning s=n*(u*n)) in pressure model

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@7429 2fb0f335-1f38-0410-981e-8018bf24f1b0