That's a very elegant solution for deprecating functions. We could surely use it in qutip. For these  functions here, I'm not sure deprecatation is the right thing though. I would certainly not recommend using this function based API, but rather use the OO API through Qobj, but for backwards compatibility these functions will probably never be removed, and some users might want to use these functions, and then should probably be able to do so without warnings..

There are other functions in qutip that are really deprecated and will eventually be removed, and then the `@deprecated` decarator could be used.