Move dummy argument `i_slice` of function `comp_ishape` to last
position. Because we are going to add a default value.

Move functions `node_to_date_eddy` and `date_eddy_to_node` from script
`report_graph.py` to script `util_eddies.py`. `util_eddies.py`
contains more basic functions, it only imports
shapefile. `report_graph.py` imports networkx. It was not convenient
nor meaningful to require networkx for scripts that use
`node_to_date_eddy` or `date_eddy_to_node`.

This was polluting standard output. So we have to add the shapefile
path when it is not found.

OSError is more specific than SystemExit. It is better to have a more
specific exception when we want to catch it.

Invert order of dummy arguments orientation and layer of method
`get_reader`. Take the same order than in the path. `orientation` was at
the end because, at first, in commit d858fb72, it was an optional
argument.

Check that `SHPC_dir` exists at initialization of instance of class
SHPC. Before this commit, if `SHPC_dir` did not exist, there was an
error trying to create `n_slices.txt`, which was less clear.

Move the second test on `self._ishape_last[i_slice][orientation]`
inside the first test. Avoid the double test on
`self._ishape_last[i_slice][orientation]` when
`self._ishape_last[i_slice][orientation]` is not None on entry of the
function. If `self._ishape_last[i_slice][orientation]` is not None on
entry of the function then, necessarily, the assertion on
`self._ishape_last[i_slice][orientation].size` and the definition of
`self._e_max[i_slice][orientation]` have already been done because
`self._ishape_last[i_slice][orientation]` and
`self._e_max[i_slice][orientation]` are only defined from the function
get_ishape_last.

In function `comp_ishape`, return None instead of crashing if eddy
index is greater than max. This allows the script
`write_date_range.py` to work with an input SHPC containing a date
without eddies.

In function `comp_ishape`, check that `eddy_index` is in the right
range.

Preparation for a check in `comp_ishape`.

On second thought, I find the default value confusing. We are
reverting commit b328ab6f.

Allow missing `ishape_last.txt` in method `comp_ishape`. This is
useful is an SHPC is missing an orientation.

Superceded by class `SHPC_class`.

The immediate motivation comes from tests `Mat_v6_to_shapefiles` and
`Plot_eddy_contours_Mat`. `Plot_eddy_contours_Mat` needs to open the
SHPC in `Mat_v6_to_shapefiles`. `Mat_v6_to_shapefiles` does not create
`n_slices.txt` and we do not want to create `n_slices.txt` in
`inst_eddies_v6.py` because this script could be run in parallel for
several slices. There is no simple way to create `n_slices.txt`
automatically after runnning `inst_eddies_v6.py` in the test
`Mat_v6_to_shapefiles`. We cannot use a redirection in a command of
the test so something like `echo 0 > n_slices.txt` would not work.

Note that orientation = None is passed to `get_ishape_last` where it
becomes `self.def_orient`.

Move function `comp_ishape` to method of SHPC class.

Several scripts allow `ishape_last.txt` not to exist. For example
`eddy_dump.py`.

The code is adapted from `filter.py`.

Especially, we want to allow missing layer `max_speed_contour` when
running script `plot_eddy_contours.py`.

The motivation is to manage an SHPC directory without opening first
all the files inside. The class allows to open files dynamically and
remember already opened files.

`slice` is a built-in class of Python.

If `date < handler["d_init"]` then the result of the function is
silently wrong.