IDs in find_trees web service call

We should probably emit the tree_id separately (not just as a part of the `oti_tree_id` field. It would be nice if the (old) numeric study IDs were emitted with the pg_ prefix, too (just to cut down on confusion for folks who don't want to read about the history of how we used to have #'s only and now have moved to pg_ and ot_).
