It is not enough to parse the input file name and extract the date
because, when we process other dates, we have to construct file names
in the same format. So do not use `parser.parse` for the input file
name, try various formats and remember the one that works.

Method poly of shapefile writer requires a list of lists, not a list
of numpy arrays. This bug goes unnoticed in pyshp `2.1.0` but raises an
exception with pyshp `2.1.3`.

This is an almost-duplicate file of `report_graph.int`: the difference
is that it processes an edgelist with nodes identified by node index,
instead of date and eddy index. We could make `report_graph.py` more
general instead, but that would degrade its clarity. Let us just have this
almost duplicate file, pending a decision on the format of edgelist.

Move functions `node_to_date_eddy` and `date_eddy_to_node` from
`convert_id.py` to `report_graph_int.py`, where they are used.

Revert part of commit dfb78dbc. Motivation: being able to use
`plot_eddy_contours.py` on results from Matlab.

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`.

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.

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.