Fixed: The "test corpus" no longer "goes missing"

Test files now dynamically detect the path to test files, so run_tests.py can be
run from any working directory.

Closes #304
Refactor: lilypond.PartNotesIndexer becomes PartNotesExperimenter

Since this indexer can't sensibly produce a DataFrame, I'm moving it into
the experimenters module. I updated the tests. Not much else was needed.

Closes #280. Related to #281.
Refactor: interval.key_to_tuple() ==> workflow.split_part_combo()

This moves the function to where it's used, and renames to more accurately
describe what it does.

Closes #288
Refactor: lilypond.AnnotateTheNoteIndexer ==> AnnotateTheNoteExperimenter

Working toward #281