Scaper

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- Use sox flag -s for time stretching (speech mode), gives better sounding results.

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- Fixed a bug where short backgrounds did not concatenate to fill the entire soundscape.

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- Fixed a bug where the soundscapes were off by one sample when generated. Fixes bug
where generating from jams using a trimmed jams file was using the trimmed soundscape
duration instead of the original duration.
- Added a field to the sandbox that keeps track of the original duration of the
soundscape before any trimming is applied.

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- Fixed a bug with the format and subtype of audio files not being maintained in
match_sample_length.

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- Fixed a bug with generating the file names when saving the isolated events. The idx for
background and foreground events now increment separately.

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- Fixed a bug with generating docs on ReadTheDocs.