Beatwise TF matrix (or multibeat)

[Yoz0]

This PR add a new feature representation with a fixed number of frames per beats. This is an advancement of issue #154 , but on beats instead of bars.

For the user this adds :

• A configurable parameter frames_per_beat (int, default 3). The number of frames sampled for each beat.
• An argument of Feature : multibeat (bool, default False). Whether to use this representation or not.
• An in line parameter of the run_MSAF.py : -mb. To use this representation.
• A little bit of documentaion.

In the Feature class :

• self._est_multibeat_features and self._ann_mutlibeat_features where the representation will be for resp. estimated beats and annotated beats.
• self._est_multibeat_frames and self._ann_multibeat_frames with the frame index of the beats. (We remove the last beat from self._est_beatsync_frames, because we compute frames between beats)
• self._est_multibeat_times and self._ann_multibeat_times with the times in secondes of the beats.
• _compute_multibeat(self, beat_frames) to compute the index of the frames used for the representation
• _shape_beatwise(self, multibeat_features) to stack the feature representation as a beatwise TF matrix.
• It also does some refactoring of compute_beat_sync_features in order to use the same function to compute multibeat features. The padding of beat_times is moved to a new function called _pad_beats_times
• Two new FeatureTypes : est_multibeat and ann_multibeat and the appropriate code in select_features(...)
• A new Exception FramesPerBeatTooHigh raised by '_compute_multibeat()if the number of frames between two beats is lower thanframes_per_beat`.
• Tests of the functions in test_multibeat.py

Thanks for reading, and reviewing code 😃

[urinieto]

What a great PR! This is a lot, but I think this is the cleanest way without having to do a massive refactoring on metafeatures. I just left a couple of minor comments regarding documentation. I am running the tests now with Github Actions. If they pass, I will merge away after the docstring typos/requests are addressed. Thanks!

[urinieto]

Apologies, I realized I never get back on this! Some of the tests didn't pass. @Yoz0 , could you please make sure all tests pass? Once they do, I'll merge this PR. Thanks!