Add support for tree agglomeration
Add missing rename_node function
Bug fix: mistyped 'qitem' instead of 'qlink'
Update tests for tree agglomeration
Add function to remove merged boundaries
Remove merged boundaries when getting segmentation
Major bug fix: peek only at valid queue items

MergeQueue.peek() was designed to look at the next item in the merge
queue without popping it. However, the next item *could* be invalid,
and cause a merge of a valid item past threshold!

This commit fixes that by having peek() discard invalid items until a
valid one is at the front of the queue.
Change feature testing to use update node ids
Update RIG creation to use watershed, not segmentation
Update refine_post_merge_boundaries to use tree agglo
Bug fix: reset tree height after ladder agglo