Save trajectories as lists of vertex indices

This is useful to find the shortest path between the beginning and the
end of a trajectory, or to create a histogram of the number of
trajectories per component.

Trajectories/trajectories.py

@@ -215,6 +215,13 @@ t0 = t1
 
 # Save:
 
+with open("traj_vert_ind.json", "w") as outfile:
+    json.dump(traj_vert_ind, outfile, separators=(",", ":"), default=int)
+    # (We specify default because the type of vertex indices is
+    # numpy.int32, not JSON serializable.)
+
+print("Created traj_vert_ind.json.")
+
 with open("traj_segm.json", "w") as outfile:
     json.dump(traj_segm, outfile, separators=(",", ":"))