Capability of specifying background label for LabelToClusterPointIndices

[wkentaro]

Why?

We'd like to fill index of cluster indices with bg_label to keep index == label_value.

[wkentaro]

@knorth55 With this PR and #2133, we can get below pipeline

A = Point cloud
B = Image
C = Tote box
D = Object label
E = Object cluster point indices
F = Point cloud in tote
G = Labeled object bounding boxes
H = Object mask being picked

A, B = RGBDCamera()
D = FCN(B)
E = LabelToClusterPointIndices(D, bg_label=-1)
F = AttentionClipper(F, C)
G = ClusterPointIndicesDecomposer(F, E, sort_by=cloud_size)

# inside of main.l
for g in G:
  if g.label not in [shelf_label, bg_label]:
    break

H = LabelToMaskImage(D, label_value=g.label)  # g.label is set with dynparam by main.l