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.

Rename procedure `get_slice_dir` to `get_ori_slice_dir`.

Rename file `get_slice_dir.f90` to `get_ori_slice_dir.f90`.

Rename type `shpc_slice_handler` to `ori_slice_handler`.

Rename file `shpc_open.f90` to `ori_slice_open.f90`.

This is a change in the format of an SHPC. I have realized that the
program `inst_eddies` may not find any eddy at a given date and
orientation (as in the test `No_eddy`), so we cannot guarantee that
the first date present in the shapefiles corresponds to the first line
in `ishape_lat.txt`. So we store the initial date explicitly in a text
file. We could deduce the final date from the initial date and the
number of lines in `ishape_lat.txt` but we can avoid counting the
lines and just store the date range.

Else the output of `subprocess.run` comes before the print statements
in the script.