[Amibe] RandomizeGroups: Allow to tag the neighbor of a triangle
[Amibe] remesh.py: Add LengthDecimateHalfEdge again

... until we get something beter. Note that it's now later in the
process so there is a lower chance of having remaining small border
edges.
[Amibe] Add EdgesCollapserNG

Also use it in RemeshSkeleton but not yet in BeamInsertion