Jams duration for symbolic datasets

[rabitt]

We currently require Track objects to have a jams_converter method, which is required to generate jams objects which validate. In order for the jams object to validate, it needs to have a "duration" passed. For some symbolic datasets, this doesn't make as much sense, and the jams format in general is not build with symbolic data in mind. Some ideas for solutions:

1. we could remove the jams_converter requirement from Track objects for certain types of datasets, and allow it to raise a custom "NotImplemented" because it's symbolic.
2. we can keep the Track object as-is, and create a new object tailored for symbolic data, e.g. SymbolicTrack.

@PRamoneda and @nkundiushuti - I think solution 1 is the simplest, but maybe there's other custom things we want to make consistent for all symbolic data. Or do you think the current Track object is fine?

As a temporary patch, for symbolic datasets we can put a dummy value for duration, like 1.

Thanks @PRamoneda and @nkundiushuti for bringing this issue up!

[magdalenafuentes]

I'm not too familiar with symbolic datasets, duration does not make sense at all in that context? Couldn't it be obtained from a midi file or something like that? @nkundiushuti @PRamoneda

If duration or the to_jams don't make sense, then I'm down for @rabitt's solution 1. If they do and duration can be obtained from midi or so, I would modify to_jams() to to get the duration from the symbolic attribute of the track object (or whatever we decide at the end).

[rabitt]

Just to link them, the outcome of this depends on what we decide with #298 !