Tests

[giamic]

Fix tests by doing the following

1. Require music21 8.0 or more recent
2. Refactor the code to avoid circular imports
3. Remove two tests on statistical analysis of corpora
4. Use the example provided in the tests folder for tests instead of files in the corpus.

Still to do:

1. Fix chord_usage — the test (which should eventually go, I believe) is showing that some files slices_with_analysis.tsv are generated empty. We should understand why and write a specific test against this case
2. Fix github action — the previous one turned out not to work, I have updated it but I can't see the effects yet

Closes #59