Firework editing

Editing is a modification of what add_node() creates. This includes:

1. Add a new task to the end of the _tasks list of a node. A principal problem is that no new links to other nodes can be created if new data dependencies occur.
2. Remove a task from the _tasks list (the task number is the only parameter). Also here not needed links cannot be removed.
3. Change task ordering in the _tasks list of a node - a new ordering must be specified

Better, add_node() should be generalized to accept a list of tasks.