[bugfix] Avoid implicit conversion to ComposedGridFunction

Before this patch, the constructor of ComposedGridFunction is available for implicit conversion during overload resolution (but would not compile). Since there is also a function localFunction(ComposedGridFunction) this makes localFunction(t) valid for any t. While this will in general not compile, the overload is still available and thus makes t look like a grid function for concepts checks e.g. in dune-vtk.

We could make the constructor explicit, but since at least one inner function is needed, we can also check for this.