Annotation.to_jams

[rabitt]

Nothing urgent here but thinking ahead. I think we could simplify the functions in jams_utils a lot by creating .to_jams methods for the Annotation classes. This would remove the need for the individual e.g. beats_to_jams methods in jams_utils and the jams_converter can become almost trivial.

[magdalenafuentes]

On the same note, we can generalize the to_jams tests so each loader doesn't have to test it. Now it's taking quite a lot of space in the loaders' customized tests