Docs inherited from Dataset showing fields they shouldn't

[magdalenafuentes]

@PRamoneda good catch!

Multitrack functionalities are showing up in the docs of all datasets but they shouldn't. Some of them:

choice_multitrack, choice_multitrack, load_multitracks, etc

[magdalenafuentes]

@rabitt would it be enough to correct the docs_inherit by checking if multi_track is implemented in the child?

https://github.com/mir-dataset-loaders/mirdata/blob/master/mirdata/core.py#L49-L63