OMR-NED and ML training mode

Introduce new ML batch evaluation command line option: --ml_training_evaluation
Introduce new ML batch evaluation API: diff_ml_training()
Introduce new OMR-NED (Optical Music Recognition - Normalized Edit
    Distance) metric.
Add comparison of RehearsalMarks.
Improve StaffGroup comparison.
Improve Metadata comparison.
Improve stem direction comparison.
Fix crashes during visualization caused by complex-duration hidden rests.