To function `plot_single_traj`.

To function `plot_single_traj`.

`comp_ishape` can now compute `i_slice`.

Compute `i_slice` in `comp_ishape` if `i_slice` is None. Add default
value of None for dummy argument `i_slice`.

Move dummy argument `i_slice` of function `comp_ishape` to last
position. Because we are going to add a default value.

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`.

Read orientation from `expanded_traj.json`.

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

So we can better distinguish initial position from small islands on
the map.

This is useful for hacking.

The downside is that we store all duration values.

Test `args.min_duration` outside the loop on `expanded_traj`. Clearer.

Invert order of dummy arguments orientation and layer of method
`get_reader`. Take the same order than in the path. `orientation` was at
the end because, at first, in commit d858fb72, it was an optional
argument.

On second thought, I find the default value confusing. We are
reverting commit b328ab6f.

Otherwise we can have a line across the globe between two points with
longitudes close to -180° and 180°.

Simpler because that is what the plot command needs.

Because we are going to define a function.