We want to add the possibility to choose a date in the slice. Note
that the script was created when there was only a single date in the
collection of shapefiles, and the script did not really change since
then.

Preparing to add an optional argument.

Preparing the option to save the figures.

Rename dummy arguments `hshpc_(cyclo|anti)` to `h_(cyclo|anti)`. They
are not SHPC handlers, they are oriented slice handlers.

It is not an SHPC handler, it is an oriented slice handler.

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.

Rename function `process_1_file` to `process_1_date`.

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__`.

In program `eddy_graph`, get initial date for each slice from
`date_range.txt` instead of from the shapefiles.

We need to create it in test `R4_15_splitted` which will be used in
test `EG_15_3_SHPC`.

Read date from `date_range.txt` in program `inst_eddies` and update
date in `date_range.txt`at the end of execution of program
`inst_eddies`, remove date from the namelist `main_nml`.

Preparing to use a second unit.

Move file `shpc_create.f90` to directory `Common/Tests`. `shpc_create`
is no longer used in program `inst_eddies` since commit dc51b1bd.

Bug fix: add source for `test_read_snapshot` even without MPI:
`test_read_snapshot` does not use MPI.