ENH/CLN: Revisit dask.bag options added in #611

[NickleDave]

In #611 we added the ability to control how audio.to_spect parallelizes creation of spectrogram files by calling dask.bag. A user can do this with an option in the [PREP] section of the config, by adding audio_dask_bag_kwargs as an inline table.

@kalleknast reported this fixed their issue (in #580) but I have to admit I'm not sure why.

We should revisit for version 1.0.

• understand why this fix works and if there's a better way
• document this (I didn't)