Watershed
Stage1: Chunk wise watershed:
Fully parallelizable
Stage2: Gets all the chunks and decides how to connect:
It has to serially process every chunk
Stage3: Relabels based on the output of Stage1 and Stage2 to apply the actual merging.
Using "--high=0.999996 --low=0.006821 --dust=100 --dust_low=0.3 --merge=800" on snemi3d
with border::none
