Strip tracknumber from title if leading
Improve clustering by grouping by directory
Tweak to not overwrite artist if it exists
Remove debug print.
Simplify code for readability.
Move album_artist_from_path to utils

and add test.
Fix typo
Merge pull request #328 from Sophist-UK/sophist_improve_clustering

Improve clustering using directory structure if no existing album tags
Add missing whitelines + album_artist_from_path() description