Preparing to make dummy arguments longitude and latitude optional.

Directory Tests was created in commit 40a1c5aa.

Move Fortran source files used in tests of both parts, `Inst_eddies`
and Overlap, to directory Tests at root of repository.

Also split the tests, Fortran source files, Python files and JSon
files. Keep at the top level the files used in both `Inst_eddies` and
Overlap.

Motivations for the split:

- The top directory contained many files.

- It may be useful to compile only one of the two sub-projects. For
example, only `Inst_eddies` on Ciclad, where MPI 3 is not available.

Pass argument copy to `read_snapshot` rather than arguments periodic
and `dist_lim`. Motivation: do not recompute copy at each call of
`read_snapshot`. `read_snapshot` is going to be called many times in
overlap program, with always the same value of copy. And we do not
need the values of periodic and `dist_lim` separately in
`read_snapshot`.

Complete `send_snapshot` and `recv_snapshot` by sending and receiving
an entire snapshot. So we need nlon, nlat and copy in `recv_snapshot`
to allocate `extr_map`. It is better to pass arguments nlon, nlat and
copy than sending and receiving the shape of `extr_map` with MPI, of
course, since the shape is the same for all snapshots. In program
`test_send_recv`, write the entire received snaphot.

Bug fix in `Tests/CMakeLists.txt`: several targets missed
`${netcdff_INCLUDE_DIR}`.

Create procedure `write_snapshot` from code in main program
`test_read_snapshot`, because we need to do this also in program
`test_send_recv`. Note the dissymmetry with `read_snapshot`, which
does not include the opening and closing of the shapefiles. That is
because we anticipate calling `read_snapshot` several times with the
same shapefiles in program overlap.

Change order of dummy arguments of procedure `write_extr_map`. More
convenient for calling it with keywords from `write_snapshot`.

0ce6b1b4.

Write extr_map in test_read_snapshot since extr_map has been
defined. So create a procedure write_extr_map because now we want to
write extr_map both in test_local_extrema and in test_read_snapshot.

Preparing to take into account periodicity in successive_overlap: add
argument periodic to read_snapshot and extend s%extr_map in longitude
if periodic.

In program extraction_eddies, constraint on max_radius(1) is necessary
only if periodic.

Make read_snapshot more general: add argument step.

In procedure get_snapshot, remove definition of s%number_eddies
because this is already done in read_snapshot.

Promote dist_lim from local constant of successive_overlap to constant
of module successive_overlap_m because we need it in read_snapshot
(and test_successive_overlap and test_read_snapshot).