Use property map for out-degree

This seems to be a little faster, even if we do not use out-degree for
all the nodes. For `Global_1993_2023/Graph_anti`, this commit reduces the time
to define `traj_prop` from 23 s to 20 s.

Trajectories/trajectories.py

@@ -76,6 +76,7 @@ traj_vert_ind = []
 # of vertex indices.
 
 in_deg_prop = g.degree_property_map("in")
+out_deg_prop = g.degree_property_map("out")
 
 # Loop to define traj_prop:
 for n in topology.topological_sort(g):
@@ -89,11 +90,11 @@ for n in topology.topological_sort(g):
         pred = g.get_in_neighbors(n)
 
         if in_deg_prop[pred[0]] == 1 == in_deg_prop[pred[1]] \
-           and np.all(g.get_out_degrees(pred) == 1):
+           == out_deg_prop[pred[0]] == out_deg_prop[pred[1]]:
             n1 = g.get_in_neighbors(pred[0])[0]
             n2 = g.get_in_neighbors(pred[1])[0]
 
-            if n1 == n2 and g.vertex(n1).out_degree() == 2:
+            if n1 == n2 and out_deg_prop[n1] == 2:
                 # We have a splitting event at n1 immediately followed
                 # by a merging at n. Check the time interval between
                 # splitting and merging. The date of merging is the