runtime parameters are just like normal parameters, except that they
don't have a property as a fallback. Thus, they are mandatory. Any
parameter can now be either a normal parameter, or a runtime
parameter. If a parameter is queried as a runtime parameter as well as
a normal one, an exception will be thrown in debug mode.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@6488 2fb0f335-1f38-0410-981e-8018bf24f1b0