Index a tree for rapid lookups within a tree array
State: Experimental as of 0.4.0.
Indexes nodes in-place as n._leaf_index.
Returns:
dict – A mapping {node_id: TreeNode}
np.array of ints – This arrays describes the IDs of every internal node, and the ID
range of the immediate descendents. The first column in the array
corresponds to node_id. The second column is the left most
descendent’s ID. The third column is the right most descendent’s
ID.