Add child references to `TreeNode`

[folmos-at-orange]

Description

Currently the TreeNode class contains only a reference to a parent. This is very unusual for a tree structure, and while still it is possible to traverse it requires consulting the TreeNode list every time or preindexing.

Having child references would simplify the traversal for this structure.

Questions/Ideas

• After reading the object, index the TreeNode nodes with a tuple children containing the references to the child nodes.
• Indexation example

# This uses the fact that the node list is in BFS
nodes_index = {}
for node in tree.nodes:
    nodes_index[node.id] = node
    node.left_child = None
    node.right_child = None
    if node.parent_id is not None:
        parent_node = nodes_index[node.parent_id]
        if parent_node.left_child is None:
            parent_node.left_child = node
        else:
            parent_node.right_child = node

[popescu-v]

Indeed, the current TreeNode implementation only provides the minimal API surface to still be able to navigate the Tree.
Nevertheless, this feature request seems sensible as it would provide significant convenience to the users IMHO.