Ease interoperability with DDC

[tpadioleau]

We notice this pattern when needing interoperability with DDC

ddc::for_each(
        ddom,
        [&](ddc::DiscreteElement<DDim> const delem) {
            int const raw_integer = delem - ddom.front();
            not_a_ddc_container[raw_integer] = ....;
        });

it would be easier to have a loop generating the DiscreteVector like

ddc::for_each_with_discrete_vector(
        ddom,
        [&](ddc::DiscreteVector<DDim> const dvec) {
            not_a_ddc_container[dvec] = ...;
        });