Nbswave

- Added support for tempo changes in the exported track.
- Fixed an exception that could occur while trying to access a layer that doesn't exist in the song. (5)

- Fixed `ignore_missing_instruments` argument not working properly. (3)
- Fixed note pitch not taking the pitch of custom instruments into account. (4)
- Removed compatibility with Python 3.7.

- Fixed arbitrary data possibly being exported due to an improper array initialization.
- Fixed output being normalized even when it was not clipping, making the exported track louder than it should.
- Fixed track always being mixed with the default audio parameters, despite being saved correctly.
- Fixed pop in the exported track due to wrong peak calculation.
- Fixed a bug in estimating the song length, which caused the output array to be resized multiple times — a costly operation.

- Fix exception due to incorrect output data length.

- Added option to `exclude_locked_layers` from the exported track.
- Added `sample_rate` and `channels` settings to the `render_audio()` function.
- Transformed sounds are no longer held in memory all at once, dramatically reducing memory usage while exporting a track.
- Fixed a `MemoryError` occurring when exporting songs with a lot of notes.
- Fixed a bug in song length calculation on NBS versions 1 and 2.

- Changed static typing of data structures to `typing` generics, adding compatibility with Python 3.7-3.8. (1)
- Fixed an argument mismatch on the `render_audio` function.