Handle (or ignore) "0" assignments from the cleave server.

[stuarteberg]

Currently, the cleave server sends back label assignments for only those supervoxels which actually ended up with a non-zero label:

{
  "body-id": 99,
  "assignments": {
    "1": [10,11,12],
    "2": [13,14,15]
  }
}

In the event that some supervoxel ends up with no label, I would like to return unassigned supervoxels as being assigned to "0":

{
  "body-id": 99,
  "assignments": {
    "0": [20,21,22],
    "1": [10,11,12],
    "2": [13,14,15]
  }
}

For now, you can simply ignore that list. In the future, we might want to do something with them (e.g. display extra error information if those supervoxels have no meshes associated with them.

[stuarteberg]

Update: The current version of the cleave server does include the "0" assignments as described above. It doesn't seem to cause neu3 any problems.