you now can basically use them like properties, (but not for compile
time expressions). e.g. the gravity may be retrieved using

GET_PARAM(TypeTag, bool, EnableGravity)

then, the value is taken from the parameter tree defined by the
ParameterTree property with the default being the value of the
EnableGravity property from the Dumux property system.

This mechanism is still subject to change and also needs proper
documentation...

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@6416 2fb0f335-1f38-0410-981e-8018bf24f1b0