Allow storage of a BindableGridFunction by reference if explicitly requested.

The usage case is as follows: one stores a BindableGridFunction in, e.g., a model class as a copy. Then the BGF is wrapped in to a ConstLocalFunction in order to add the CLF interface. This makes a second copy. However, this second copy is completely useless as the model already holds a copy and is also a "bindable object". In this case we only need the LF-interface methods, but not the additional storage.