Autonumbering is fragile

At present the creation of a node with no `nid` parameter causes autonumbering of the node. The creation of a new tree therefore requires a `Node.reset` call, but sadly this will mess up the autonumbering if further nodes need to be created in an already-existing tree.

The solution is to replace the current `Node.reset` calls with a call to create a new tree, and to arrange that nodes know which tree they are a part of.