We want to be able to re-compute the cost, starting from the output of
a previous run of `cost_function.py`, when we have discarded the input
file of the previous run.

Segments is no longer a test with only isolated segments since commit
e1c18ad8.

Add tests `Cost_function_avg_date`, `Greece_date` and
`Eurec4A_cost_date`.

Add option `--avg_fix` in tests to reproduce previous behaviour.

The old behaviour: averaging over a maximum number of instantaneous
eddies, becomes an option.

We do not use bisect because `max_delta` should be small, around 6,
and most often there should be only one time step between
instantaneous eddies, so the first guess at distance `max_delta`
should be close to the result. Moreover, we would need to define a
date function if we used bisect.

Preparing for generalization of these functions.

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.