Because we want to be able to easily read the edgelist with several
graph programs and they would not all accept the same
header. `read_edgelist` from Networkx accepts a comment line starting
with a special character. Not very elegant to put column headers after
a #. Anyway, the headers predecessor and successor are obvious for an
edge list.

Do not crash on missing SHPC directory for interpolated
eddies. Motivation: when we call `read_eddy_graph` from
`plot_traj.py`, we just try to read data on interpolated eddies.

Motivation: there may be only one of the two edgelist files in the
current directory.

This can happen with Matlab data, if a node present in the edge list
is absent in the shapefiles.

We now call `date_index` the index k starting at 0. See documentation.

Not dependant on global variables. `date_num` is no longer relevant.

Clearer. `e_max` is now the number of eddies of the current
orientation at the current date. See documentation.

Write node index instead of date and eddy index to the edgelist
file. Motivations: stay closer to Matlab, at least for now; some graph
processing programs other than NetworkX will not easily read nodes
that are tuples. Consequences of this change are that we no longer
need `date_num` from the Matlab files and we no longer need the
functions `date_index` and `convert_id` in the script
`overlap_v6.py`. So move these functions for now to a new file
`convert_id.py`.

We follow Matlab naming. We want the transition in output format to be
as easy as possible.

Rename field `date_index` of shapefiles to `days_1950`. Clearer.

The files are now sufficiently small to be imported.

Select even more precisely the data we need in the mat files. We only
save to v6 this data. This also makes the loading script in Python a
little clearer.

Create mat v6 files containing only the variable necessary to the
script `survival.py`. Motivations: reduce time for saving to v6,
reduce storage, reduce time for loading in Python.

Both anticyclonic and cyclonic trajectories.

We just write a line in `ishape_last.txt`. Note that we cannot catch
the error, which occurs inside Matlab, so we have to test for file
existence beforehand.

Files for EUREC4A have format `adt_YYYY-MM-DD.mat` while files for
domain PhD-Lax and global domain have format `adt_YYYY-MM-DD.mat`, so we
need something more general than strptime.

Write to `SHPC_(anti|cyclo)` instead of `Snapshot_(anti|cyclo)` since
we are now writing several dates.

Create and use module FindNetCDF_Fortran, modeling on FindNetCDF.

Move the contents of these directories up one level, renaming them.

Instead of moving it to `Tests/Region_1_SHP_triplet_noise`. Two
reasons:

- It is annoying to have a JSon file in the tree which we do
not want to include in the tests.

- It does not correspond completely to what is in
`Tests/Region_1_SHP_triplet_noise`. We have four dates in
`Tests/Region_1_SHP_triplet_noise`.