fix: isocrones are not taking into account the time

[pareyesv]

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According to the guidelines above, I think this issue proposes specific changes to existing examples.

Regarding 13-isolines-isochrones.ipynb, the following cell:

# add an edge attribute for time in minutes required to traverse each edge
meters_per_minute = travel_speed * 1000 / 60  # km per hour to m per minute
for _, _, _, data in G.edges(data=True, keys=True):
    data["time"] = data["length"] / meters_per_minute

the cell is not assigning the attribute time to the edges in G.

Then, ego_graph is not taking into account time:

for trip_time, color in zip(sorted(trip_times, reverse=True), iso_colors):
    subgraph = nx.ego_graph(G, center_node, radius=trip_time, distance="time")
    for node in subgraph.nodes():
        node_colors[node] = color

Actually, ego_graph should fail, but it doesn't raise an error.

Consider changing the for-loop cell with the following:

# add an edge attribute for time in minutes required to traverse each edge
meters_per_minute = travel_speed * 1000 / 60  # km per hour to m per minute
time_attr = {(x, y , z): data["length"] / meters_per_minute 
                            for x, y, z, data in G.edges(data=True, keys=True)}
nx.set_edge_attributes(G, time_attr, name="time")

[gboeing]

the cell is not assigning the attribute time to the edges in G

Yes it is. You can see the new time edge attributes if you run this line after that cell:

nx.get_edge_attributes(G, "time")