GitLab

* Addressed review issues in PR #668

* Changed GTZAN so that it only traverses filenames belonging to the dataset

Now, instead of walking the whole directory and subdirectories of the dataset
GTZAN only looks for files under a `genre`/`genre`.`5 digit number`.wav format, where `genre` is an allowed GTZAN genre label.
This allows moving or removing files from the dataset (e.g. for fixing duplication or mislabeling issues).