Bug fix: start list `inst_eddies` with vertex name

Initialize the list `inst_eddies` with vertex name in the initial
graph, which is the graph of eddies. Else the list for a segment may
miss some instantaneous eddies. This bug is demonstrated in test
`Edgelist_Isic_2` where segment 10 missed eddy 10 and segment 29
missed 29.

Trajectories/segments.py

@@ -37,13 +37,11 @@ t0 = t1
 
 g.vp['inst_eddies'] = g.new_vp('vector<int>')
 print('Collapsing into segments...')
+for v in g.vertices(): g.vp.inst_eddies[v] = [int(g.vp.name[v])]
 g.set_fast_edge_removal()
 verts_to_del = []
 
 for v in g.vertices():
-    if not g.vp.inst_eddies[v]:
-        g.vp.inst_eddies[v] = [int(g.vp.name[v])]
-
     if v.in_degree() == 1:
         v2 = next(v.in_edges()).source()