Better Transform for Vdicts

[thomasahle]

Description of proposed feature

It would be useful if using Transform on VDicts matched items according to their keys. Basically this:

def transform_vdicts(scene, vdict1, vdict2, run_time=1):
    k1 = vdict1.submob_dict.keys()
    k2 = vdict2.submob_dict.keys()
    transforms = []
    for key in k1 - k2:
        transforms.append(FadeOut(vdict1[key]))
    for key in k2 - k1:
        transforms.append(Create(vdict2[key]))
    for key in k1 & k2:
        transforms.append(Transform(vdict1[key], vdict2[key]))
    scene.play(*transforms, run_time=run_time)

Right now, I think, Transform just looks at the order of the items, ignoring the keys. Basically it converts the Vdict into a Vgroup.

How can the new feature be used?

I have a graph, and I store each edge in a vdict:

def graph_to_vdict(G):
  vdict = VDict()
  pos = nx.shell_layout(G)
  for u, v in G.edges:
      vdict[i, j] = Line(start=pos[u], end=pos[v])

If I have a graph, and add/remove some edges, it's important for the Line objects to be matched up with the same mobjects as before. Otherwise everything will move around unnecessarily.