Transactional adds/removes

As a compromise between the status quo and #12, what about adding a transactional add/remove of nodes and edges?

For example, some code like this:

for node_ind in nodes_to_remove.iter() {
  tree.remove_node(*node_ind);
}
for edge_ind in edges_to_remove.iter() {
  tree.remove_edge(*edge_ind);
}

As I understand it, this code has somewhat random behavior since each run through the loop will cause the tree to change. However, if I could specify something like tree.transact(&nodes_to_remove, tree.remove_node), that would solve my own stability needs without needing the whole graph to be redone.

On the other hand, it's a very database thing to do.