this is primary to allow models to check whether their assumptions are
valid for the given fluidsystem

also add the H2O-Air fluid system to the fluid systems test. this
caught a few issues!

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@7537 2fb0f335-1f38-0410-981e-8018bf24f1b0