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 `process_1_file` to `process_1_date`.

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

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.

Rename procedure `get_slice_dir` to `get_ori_slice_dir`.

Rename type `shpc_slice_handler` to `ori_slice_handler`.

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.

Clarifying the logic: the Fortran program only appends to an already
created SHPC slice.

Use `create_slice.py` in tests of program `inst_eddies`. The namelists
then have to be written to a file, to be read both by
`create_slice.py` and by the program `inst_eddies`.

The program `test_mean_speed` no longer needs the SHPC directory since
commit 8eadc458.

Call `create_slice` from script `inst_eddies_INALT.py`, instead of
just removing and creating the directories. Note that we then need to
know `uniform_lon_lat`.

Instead of just removing and creating the directories. Note that we
then need to know `uniform_lon_lat`. Also note that we need to copy
the file `create_slice.py` to the build directory where it is used by
`inst_eddies.py`.

Preparing to only append to shapefiles in the Fortran program, not
create them.

Write longitude and latitude if `leave_trace` and not `uniform_lon_lat`.

A negative value is even more striking since a step for a uniform grid
would be positive.