Audio paths when saving isolated sound events are absolute sometimes

[pseeth]

For some reason, in the FUSS data generated by Scaper, the paths that are in the JAMS files point to absolute paths on the machine used to generate the dataset. To load it using those files, we need to correct the absolute paths to be relative to the JAMS file, like I do here: https://github.com/nussl/nussl/blob/master/nussl/datasets/hooks.py#L334.

I think that the paths in the JAMS file should be relative to the location of the JAMS file. Will have to investigate how this happened as I don't see anything obvious for why it's happening right off the bat.

[justinsalamon]

I believe the current behavior is to document absolute paths (at the time there wasn't a use case for this feature other than documentation and completeness so I didn't think it through too heavily).

Storing paths relative to the JAMS file sounds like a great solution to me.

@pseeth is this on the critical path of our current endeavors?