extend annotations functionality

Annotations now support time ranges, snapping to trajectories and exporting to JSON. Together with the loading/saving functionality of bb_behavior this now constitutes a potent tool for behavior labeling.
Also, please don't abbreviate functions like that (see squashed commit).

This commit squashes
https://github.com/walachey/biotracker_core/commit/509ee990e2d3b0bc6a19be86f622c813226e0abe
https://github.com/walachey/biotracker_core/commit/317153ab48cb371a2ec49a4572dd4afe3e441219
https://github.com/walachey/biotracker_core/commit/6a28b02a3d007303ae1fda1202746d9988a4f753

I would have rebased cleanly but the frequent refactoring makes this a PITA.