Make TrajectoryCollection indexable

[raybellwaves]

Give a TrajectoryCollection you can index it using traj_collection.get_trajectory(traj_id_col VALUE).

I would like to be able to do traj_collection[0] which would do unique traj_id_col VALUES[0].

This will get a bit tricky if you have values "z", "y", "x". Should 0 return "x" or "z".

Either way I think mostly for exploration whatever the order of .unique() returns can be used. get_trajectory should still be encouraged but I feel a __getitem__ will help with quick exploration

[anitagraser]

Hi Ray. Could you elaborate how this differs from TrajectoryCollection.tajectories[0]

An example would be great to help me understand