Automatically remove nodes from parents

Automatically remove nodes from parents

After removing a node from a WakeupTree,
we need to be careful to also have any references
to the node removed, viz. that held onto the
parent of the node in the list of children.
This commit adds a back reference to the
parent node of a given WakeupTreeNode to
allow removal from the parent when destroyed.

Additionally, some bugs were fixed for creating
subtrees and removing subtrees (we forgot to remove
the root for example)