Updated the audio IO package.

Updated the audio IO package.

Fixed crosshair bug in UX when the spectrogram types have very differ…

…ent DTWs.

Fixed a casting bug in ux.cc.

Added support for producing approximate mean opinion scores.

Also added the possibility to produce unified output for all channels of a sound.

Minor improvements.

- Changed default frequency resolution from 8 to 5, which seems to perform better in general.
- Made Study.Put add all a slice of references in the same transaction, which improves performance a lot when using remote drives.
- Made  print all references in the dataset in a single JSON array instead of individual JSON dictionaries to simplify ingestion via JSON.
- Made progress bar use regular average instead of EMA the first 10 seconds.
- Made progress bar use fractional speed instead of completed-units-speed when predicting ETA, since that automatically takes changing number of pending tasks into account.
- Made progress bar able to finish with an actual time of completion.

Made the progress bar not crash tty-less terminals.

Updated notebooks.

Made Zimtohrli work with either GNU or LLVM toolchains.

Also:
- Added a bit of README to the go code.
- Made the Go wrapper link against stdc++, since that is more common.

Enabled skipping building Zimtohrli tests.