Split supervoxel from all other segments

[philippotto]

The proofreading tool currently supports split and merge operations. We should also add a "Split from all other segments" operation to the context menu. The supervoxel that was rightclicked should then have no edges in the editable mapping, anymore.

[fm3]

I’d propose a new route similar to min-cut, called neighbors,

that takes

• a position
• a mag
• an agglomerate id

and returns

• the segment id at that position
• the set of direct neighbors of that segment in the graph, specified by
  • their segment id
  • their position

Then the frontend can construct split update actions that together should perform the desired operation. What do you think @philippotto ?

[philippotto]

As discussed in person: sounds good!