Track.get_track_radio and Artist.get_radio should return a Mix or the mix id

[Nokse22]

Looking at the API calls in the browser I can see that they use https://tidal.com/v1/artists/3514310/mix to get the radio of an artist, same for tracks, here are two examples.
https://tidal.com/v1/artists/3514310/mix returns {"id":"000038b3b74d5ce3a17b43a36d62bb"}
https://tidal.com/v1/tracks/12445712/mix returns {"id":"001c2cbc32b5b7c17f8c0aa55d9541"}

Maybe we could add Track.get_radio_mix and Artist.get_radio_mix so not to break the API.

I have tried to add this

[tehkillerbee]

Fixed in #330

We could consider removing the legacy getters - or perhaps mark them for deletion in a future release.