Create a SubDomain

[EmilyBourne]

I would like to be able to create some kind of SubDomain. For example this could be achieved by adding a stride to the Domain. This would be useful in several places.

It can be used to iterate over a sliced domain. This can be useful for creating a Lagrange interpolation or calculating a quadrature (e.g. Simpson's quadrature).
E.g. currently a Simpson's quadrature is written as:

    for (auto it = domain.begin() + 1; it < domain.end() - 1; it += 2) {
        ddc::DiscreteElement<IDim> idx = *it;
        coefficients(idx) = 2. / 3. * (distance_at_left(idx) + distance_at_right(idx));
        idx += ddc::DiscreteVector<IDim>(1);
        coefficients(idx) = 1. / 3. * (distance_at_left(idx) + distance_at_right(idx));
    }

with a SubDomain it would be:

    for (ddc::DiscreteElement<IDim> idx : subdomain) {
        ddc::DiscreteElement<IDim> idx2 = idx + ddc::DiscreteVector<IDim>(1);
        coefficients(idx) = 2. / 3. * (distance_at_left(idx) + distance_at_right(idx));
        coefficients(idx2) = 1. / 3. * (distance_at_left(idx2) + distance_at_right(idx2));
    }

It can also be used to create a Chunk on a SubDomain. Eg when working with patches it is common to need values of a function over all the domain, but the derivatives only at certain points (usually the boundary). Currently this is only possible by explicitly choosing the number of points at the compilation, or creating a separate mesh for the derivatives (which then have different DiscreteElements)