graph.py: compute edge risk only when not minifying arbors.
graph.py: compute edge risk only when not minifying arbors.

fzadow/CATMAID,fzadow/CATMAID,htem/CATMAID,fzadow/CATMAID,fzadow/CATMAID,htem/CATMAID,htem/CATMAID,htem/CATMAID