refactor TreeNode::group_nodes_below

[shawnlaffan]

TreeNode::group_nodes_below is pretty complex for what it does.

It might be possible to refactor it using a Schwartzian transform style approach where the bounds and node ref are stored in an array of arrays. That in turn might need a binary search approach if we keep things in sorted order, which might slow things down.

[shawnlaffan]

Initial updates have been done as part of #803