pass `pbar` to `iterator(pbar: Graphband)`

[PythonFZ]

Allow for

def graph_tasks(pbar: Graphband):
    # allow for with pbar.lock here
    digraph = nx.DiGraph()
    edges = [
        ("a", "b"),
        ("a", "c"),
        ("b", "d"),
        ("b", "e"),
        ("c", "f"),
        ("c", "g"),
    ]
    digraph.add_edges_from(edges)
    for node in nx.topological_sort(digraph):
        yield Task(
            id=node,  # unique string representation of the task
            data=node, # optional data associated with the task
            dependencies=set(digraph.predecessors(node)), # dependencies of the task
        )

for task in Graphband(graph_tasks()):
    ...