The node identifiers in Matlab use the date index in the array of
processed dates. This is not simply the difference between the current
date and the first date if there are missing dates. This means that
converting between node index in Matlab and date and eddy indices, one
needs the array of processed dates. Morever, the conversion from date
and eddy indices to node index requires a search through the array of
dates. So we do not keep the Matlab node identifier. We use k and n as
described in the documentation.

For this commit, we need to revert in part commit 68da90cd. We have to
read `date_num` from the Matlab files, and we create function
`adjust_n` to convert from Matlab node identifier to the new node identifier.

Rename function `convert_id` to `node_to_date_eddy`.

No longer useful. Replaced by `Inst_eddies/inst_eddies.py`.

New script to loop on detection of instantaneous eddies.

I could not reproduce the old run. And we need to update this input
directory because of the change of DBF field `date_index` to `days_1950`.

Makes it easier to log. This does not make more instructions because
 now we do not need to try creating `SHP_triplet/extremum.dbf`.

The bug comes from commit ce139a86.

We have to precise the suffix because we test readability of file with
`os.access` below. The bug comes from commit 41c8c5cc.

The bug comes from commit 4ff019ac.

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.