It was not convenient to read them with input function or on the
command line: too much to read this way, and not clear when saved in
JSON text file. And we already had to create a namelist file
anyway. And this way the namelists can be extended transparently.

Pass a namelist object instead of a string to functions
`loop_inst_eddies` and `process_1_file`. Preparing to integrate the
date and the slice to this object.

Read `inst_eddies_nml.txt` out of function
`loop_inst_eddies`. Preparing to modify the date in the namelist in
`inst_eddies_Aviso.py`.

Instead of deducing it from the longitude coordinate. The deduction
could not be generalized to a non-uniform longitude-latitude
grid. Also, the test on `step(1)` and nlon was not perfectly robust.

Add argument SHPC-dir to script `inst_eddies_Aviso.py`. We want to be
able to run `inst_eddies_Aviso` in parallel for several slices. Since
we write `used_config_nml.txt` to the current directory, we need to
run the parallel instances from different directories. So the current
directory cannot always be the directory containing the SHPC to be
appended to.

Add argument `SHPC_dir` to function `loop_inst_eddies`. We want to be
able to run `loop_inst_eddies` in parallel for several slices. Since
we write `used_config_nml.txt` to the current directory, we need to
run the parallel instances from different directories. So the current
directory cannot always be the directory containing the SHPC to be
appended to.

We rewrite `inst_eddies_Aviso.py` so it is more convenient to use. The
script is only useful for several input files. Otherwise we can
directly use `inst_eddies.py`. So instead of optional arguments
template and last date, we use positional arguments. Then there is no
point in distinguishing dirname and basename. Also, it does not make
sense any longer to read the date from the content of the first
file. The script becomes more general: we do not assume that all input
NetCDF files are in the same -directory and that the time coordinate
is in days since 1950-1-1. The interface is also clearer.

This is useful because, for the year 2021, the last part of Aviso file
names varies.

Define it inside the function since we only need it here and we
already have the argument `first_file`. Simpler and clearer.

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

Assume that a slice is in a directory
"{shpc_dir}/{orientation}/Slice_{slice}" where shpc_dir can be any
name, orientation is "Anticyclones" or "Cyclones" and slice is an
integer. Do not read or create any longer a file "orientation.txt" in
the slice directory. Add procedure `get_slice_dir`. Add dummy
arguments cyclone and slice to procedure `shpc_open`. Add dummy
argument slice to procedure `shpc_create`.

We need this since `inst_eddies_Aviso.py` imports `inst_eddies` which
is in the binary directory.