Scaper

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- Added a new distirbution tuple: ``("choose_weighted", list_of_options, probabilities)``, which supports weighted sampling: ``list_of_options[i]`` is chosen with probability ``probabilities[i]``.

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- Scaper.generate now accepts a new argument for controlling trade-off between speed and quality in pitch shifting and time stretching:
- quick_pitch_time: if True, both time stretching and pitch shifting will be applied in quick mode, which is much faster but has lower quality.

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- Scaper.generate now accepts two new optional arguments for controlling audio clipping and normalization:
- fix_clipping: if True and the soundscape audio is clipping, it will be peak normalized and all isolated events will be scaled accordingly.
- peak_normalization: if True, sounscape audio will be peak normalized regardless of whether it's clipping or not and all isolated events will be scaled accordingly.
- All generate arguments are now documented in the scaper sandbox inside the JAMS annotation.
- Furthermore, we also document in the JAMS: the scale factor used for peak normalization, the change in ref_db, and the actual ref_db of the generated audio.

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- Switching from FFMpeg LUFS calculation to pyloudnorm for better performance: runtime is reduced by approximately 30%
- The loudness calculation between FFMpeg LUFS and pyloudnorm is slightly different so this version will generate marginally different audio data compared to previous versions: the change is not perceptible, but np.allclose() tests on audio from previous versions of Scaper may fail.
- This change updates the regression data for Scaper's regression tests.
- This release used soxbindings 1.2.2 and pyloudnorm 0.1.0.

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- Trimming now happens on read, rather than after read. This prevents the entire file from being loaded into memory. This is helpful for long source audio files.
- Since the audio processing pipeline has changed, this version will generate marginally different audio data compared to previous versions: the change is not perceptible, but np.allclose() tests on audio from previous versions of Scaper may fail.
- This change updates the regression data for Scaper's regression tests

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- Uses soxbindings when installing on Linux or MacOS, which results in better performance.
- Adds explicit support for Python 3.7 and 3.8. Drops support for Python 2.7 and 3.4.