This does not change any result, it is just an equivalent
formulation. We are preparing for a generalization of the choice of
`ip_beg` and `ip_end`.

Return `i_slice` instead. Since `i_slice` is now computed inside
`select_ishapes`.

Since getting `i_slice` is now cheaper.

Return `i_slice` instead. Since getting `i_slice` is now cheaper.

Instead of defining `self._n_dates` with zeroes. So we do not need the
function `get_n_dates` any longer.

Do not define `self._ishape_last[i_slice][orientation]` in
`_create_ishape_last`. Return a numpy array instead. Clearer.

`comp_ishape` can now compute `i_slice`.

Compute `i_slice` in `comp_ishape` if `i_slice` is None. Add default
value of None for dummy argument `i_slice`.

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

Except with option `-i`, which still assumes slice 0.

For `Test_compare` version > 0.4.

The algorithm of `eddy_graph` assumes that the set of dates it
processes is evenly spaced.

Because we are also going to use it to check continuity of dates
across slices.

We do not want a symbolic link `$tests_old_dir/Extract_component_nx_2`
to be created if directory `$tests_old_dir/Extract_component_nx_2`
does not exist.