ENH: 添加对隐藏可调用容量或权重参数边的支持

G = nx.DiGraph()
G.add_edge(0, 1, weight=1)
G.add_edge(1, 2, weight=2)
def myweight(u, v, e_attr):
    if u == 0 and v == 1:
        return e_attr['weight']
    else:
        return None
nx.bellman_ford_predecessor_and_distance(G, source=0, weight=myweight)