Extract code from program `inst_eddies` into new procedure `write_snapshot`.

Rename procedure `write_snapshot` to `save_snapshot`.

Move `Common/write_snapshot.f90` to `Overlap/Tests/save_snapshot.f90`.

Extract code from program `inst_eddies` into new procedure `input_ssh`.

Split derived type `shpc_slice_handler` into `shpc_slice_handler` and
`shpc_slice_meta out`. Do not mix what is necessary to open, create
and close, analogous to a logical unit, with what comes from reading
the files.

Because we are going to use it in `Inst_eddies` too.

Create component `unit` for `ishape_last.txt` in derived type
`shpc_slice_handler`. Open, create and close `ishape_last.txt` at the
same time we open, create and close the shapefiles. So we use
`read_opcol` instead of `read_column` when we need to read
`ishape_last.txt`.

We are more clearly separating opening files and reading from them.

This is no longer needed since commit d16d3f1c.

Use `e_overestim.txt` instead of `node_id_param.json`. Since commit
43d2eff6, we wrote only `e_overestim` to the JSon file. Simpler to
write the value alone in a text file.

Option should have been updated in commit e35226da.

Bad names from commit c45cd728.

Following renaming of script.

Add script converting Graph-tool output to edgelist. So we can use
analysis scripts developed in Overlap on results of trajectory scripts.

Move tests on trajectories to new test file `Trajectories/tests.json`.

Directory Tests was created in commit 40a1c5aa.

Rewrite following an idea of R\'emi Laxenaire: loop on edges in order of
ascending cost function.

Rename trajectories to `traj_segm` and `expanded_trajectories` to
`expanded_traj`.