Ignore properties of input graph

We want to be able to re-compute the cost, starting from the output of
a previous run of `cost_function.py`, when we have discarded the input
file of the previous run.

Trajectories/cost_function.py

 #!/usr/bin/env python3
 
-"""This script takes the graph of segments without cost and computes
-the cost applied to edges.
+"""This script takes a graph of segments and computes the cost applied
+to edges.
 
 Input:
 
--- the graph of segments without cost;
+-- the graph of segments;
 
 -- the SHPC.
 
 Output: the graph of segments with cost.
 
 The inst_eddies property of vertices is not modified by this
-script. All the content of the input graph is part of the output graph
-so the input file may be removed, if desired, after running this
-script.
+script. The cost is an internal property of the output graph. This
+internal property is created if it was not already in the input
+graph. The cost is overwritten if it was already in the input graph.
 
 """
 
@@ -217,7 +217,18 @@ delta_r_std = 8.6953e3  # in m
 # Load the graph_tool file:
 
 print("Loading graph...")
-g = graph_tool.load_graph(args.input_segments)
+g = graph_tool.load_graph(
+    args.input_segments,
+    ignore_vp=[
+        "pos_first",
+        "pos_last",
+        "first_av_rad",
+        "first_av_ros",
+        "last_av_rad",
+        "last_av_ros",
+    ],
+    ignore_ep=["cost_function"],
+)
 print("Loading done...")
 print("Input graph:")
 print("Number of vertices:", g.num_vertices())