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.

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.

Do not try to plot grid bounding box if not `uniform_lon_lat`, for
now.

Only write `used_config_nml.txt` and `grid_nml.txt` if `leave_trace`.

Promote u and v to variables of module `input_ssh_m`, and remove them
as dummy arguments of `input_ssh`, `set_contours`, `mean_speed` and
`set_max_speed`. u and v are defined in procedure `input_ssh` and used
only in procedure `mean_speed`.

Remove dummy argument corner of function `mean_speed`, not used any
longer.