Use argparse

Because we want to add an option.

Trajectories/draw_segments.py

@@ -9,14 +9,16 @@ found then the trajectories are colored in red.
 """
 
 import graph_tool
-import sys
 import pygraphviz as pgv
 import json
+import argparse
 
-if len(sys.argv) != 3:
-    sys.exit("Required arguments: input-Graph-tool-file output-Graphviz-file")
+parser = argparse.ArgumentParser()
+parser.add_argument("input_file", help = "Graph-tool file")
+parser.add_argument("output_file", help = "Graphviz file")
+args = parser.parse_args()
 
-g_in = graph_tool.load_graph(sys.argv[1])
+g_in = graph_tool.load_graph(args.input_file)
 g_out = pgv.AGraph(directed = True)
 
 for v in g_in.vertices():
@@ -42,7 +44,7 @@ else:
             e = g_out.get_edge(segment_list[i], segment_list[i + 1])
             e.attr["color"] = "red"
 
-g_out.write(sys.argv[2])
+g_out.write(args.output_file)
 
 g_out.close()
 # strange failure of the script without this, except in ipython: