Instead of symbolic links. We want to avoid creating symbolic links
above SHPC because we want to run several instances of `inst_eddies`
in parallel.

So that several processes running `inst_eddies` for different slices
can run in parallel.

So that several processes running `inst_eddies.py` for different
slices can run in parallel.

So the script can write to a slice other than 0.

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.

For reproducibility.

Because we are going to add an optional argument.

Add arguments `input_segments` and `output_segments`. Because on
one occasion I wanted to use and keep two versions of input and output
graphs in the same directory

Note that `traj_prop[source] == traj_prop[target]` at this line.

So we do not have to symlink the file `traj_segm.json`.

Because we want to add an option.

This is required to read an edgelist coming from `eddy_graph`.

Bug fix: extend `inst_eddies` of predecessor with `inst_edddies` of
circumvented node, instead of just name of circumvented node. The bug
is demonstrated in test `Edgelist_Isic_2` where segment 10 missed eddy
12 and segment 13 missed eddy 15.

Initialize the list `inst_eddies` with vertex name in the initial
graph, which is the graph of eddies. Else the list for a segment may
miss some instantaneous eddies. This bug is demonstrated in test
`Edgelist_Isic_2` where segment 10 missed eddy 10 and segment 29
missed 29.

It is useless to add to an attribute to v since, after the current
iteration, v will be isolated, and so it will not be used in remaining
iterations, and moreover it will be deleted after the loop.

Do not delete delete v just because it is isolated. v may have become
isolated at an earliest iteration because it is the first eddy in an
isolated segment. The bug is demonstrated with test `Edgelist_Isic_2`,
where vertex 29 was erroneously deleted.