Avoiding multiplication by 1000 for each instantaneous eddy.

Preparing to keep the radius in km and integrate a factor 1000 in
`FACTOR`.

To avoid repeated computation.

Rename function `calculate_radii_rossby` to `avg_radii_rossby`.

Take three lists as arguments of function `calculate_radii_rossby`
instead of a list of dictionaries, since this is what we have on the
calling side as vertex properties. Note that we no longer have to
distinguish the case where `ip_beg <= ip_end` in the main part of the
script.

Return a tuple of lists from function `node_to_prop`, instead of a
list of dictionaries, since this is what we need to store as
properties of a vertex.

Preparing to move long tests to a separate file.

Use vertex properties.

Preparing to compute statistics on the variation of these properties.

Instead of `in_edges` followed by `source`. `in_neighbors` is
equivalent but more direct.

In method `__init__` of class `SHPC_class`. As a consequence, we
remove the dummy argument `def_orient` of `__init__`.

It is now the same as test `Draw_component`.

Advantages: no need to provide the JSON file on the command line;
single loop on nodes.

Keep trajectory lists inside dictionaries, indexed by trajectory
number, instead of lists. This way, we can keep the trajectory numbers
when we work on part of the graph, for example in a component.

Useless since commit d95e4619.

The motivation is to be able to create the files `*traj*.json` for
only a component of the graph, so that `plot_traj.py` can plot the
trajectories in a single component, with different colors for
different trajectories.

To function `plot_single_traj`.