Nm planner - fix this api

Prefere `const auto&` when possible to avoid useless copy.

Can't we stop this double loop when this test is true? Musn't this test be true at least one time?

If yes to these, what do you think of a function (about) like that:
```c++
static const nt::EntryPoint&
get_n_item(const nt::StopPoint* stop_point,
           const std::vector<std::pair<type::EntryPoint, std::vector<std::pair<type::idx_t, navitia::time_duration>>>>& n_calc_dep) {
    for (const auto& n_point: n_calc_dep)
        for (const auto& n_stop_point: n_point.second)
            if (stop_point->idx == n_stop_point.first)
                return n_point.first;
    throw navitia::exception("RAPTOR::compute_nm_all::get_n_item: not found!");
}
```
like that, we stop the loop earlier, if we don't find, we know it (and we don't need the guard above).