introduce overlay fluid states for pressure, temperature and saturation

an overlay fluid state takes an arbitrary other fluid state as an
argument to which it forwards everything except the quantity which it
overlays.

also, the fluidsystems test now tests all fluid states for API conformance...

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@7110 2fb0f335-1f38-0410-981e-8018bf24f1b0