add methods to retrieve from annotation data instances

[csweaver]

Is there a reason that the annotation type is left out of libdvid? Admittedly I can use the custom_request function but I like the idea of not having to know the exact api endpoint.

This is probably not a high priority request.

[stephenplaza]

annotation was a newer type added to DVID after a lot of the initial code was written for libdvid. In theory, we should be adding interface as DVID expands. Time limitations.