These files will usually be created in the same directory.

This is useful to find the shortest path between the beginning and the
end of a trajectory, or to create a histogram of the number of
trajectories per component.

Move functions `node_to_date_eddy` and `date_eddy_to_node` from script
`report_graph.py` to script `util_eddies.py`. `util_eddies.py`
contains more basic functions, it only imports
shapefile. `report_graph.py` imports networkx. It was not convenient
nor meaningful to require networkx for scripts that use
`node_to_date_eddy` or `date_eddy_to_node`.

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.

We know that we are going to need in-degree for every node of the
graph. For `Global_1993_2023/Graph_anti`, this commit reduces the time
to define `traj_prop` from 31 s to 25 s.

This looked good but was much longer. For example, for
`Global_1993_2023/Graph_anti`, this commit reduces elapsed time
approximately from 24 mn to 2 mn.

This makes quite a difference in file size for a large domain. For
example, the size of `Global_1993_2023/Graph_anti/expanded_traj.json`
is reduced from 0.6 to 0.3 GiB. We do not need pretty display for
these files.

This is convenient for scripts `plot_traj.py`, `filter_traj.py` and
for hacking with function `get_duration` of `plot_traj.py`.

This is case 2 correction in the Matlab program of Rémi Laxenaire.

The test is useless in this version since we know `traj_prop[n] == -
1` at the beginning of the iteration, but we are preparing to process
phantom patterns.

Preparing to process phantom patterns.

Define `traj_segm` and `expanded_traj` after the loop on
vertices. This is clearer, and we are preparing to process phantom
patterns.

Preparing to define `traj_segm` and `expanded_traj` out of the loop on
vertices.

Change type of vertex property name in graph-tool graphs from string
to int. The option used in function `load_graph_from_csv` requires
version >= 2.49 of graph-tool.

The old algorithm did not give correct results on tests
`Greece_trajectories`, `Test_order_edges` and `Traj_component`. We
abandon the idea of processing edges in date order. Instead, we now
process vertices in topological order.

`my_subgraphs` is only used inside the test on `"cost_function"`.

Instead of reading it from standard input.

The idea is to favor long trajectories.

Preparing for more complex script.

Extract code from the main part of `trajectories.py` into new function
`assign_traj_edge`. Because we are going to make the script more
complex.

If there are no edges in the graph of segments then there are no cost
values. In this case, in recent versions of graph-tool, when
graph-tool loads the graph, it does not create a graph with the
`cost_function` property, even if the property is declared in the
input graphml file.