Don't allow nodes to push and pull outside the scope of a parent

[liamhuber]

This will play well with #668, where if we introduce Graph as a lexical parent to Node, we can move responsibility for pushing and pulling on nodes over to the parent Graph object and simplify Node.

This can be done in stages, where first we simply disable the action when the parent is not set, and later we actually migrate the responsibility and code over to the parent object (Graph or whatever it gets called)