New property names: set default to the value of the corresponding old-named property. Set defaults for old-named properties, including "true" for EnableGravity". Reviewed by Christoph.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@9187 2fb0f335-1f38-0410-981e-8018bf24f1b0