fix: set algebra

[gouarin]

Description

A new issue has been detected in the set algebra with the difference operator. This PR solves this issue.

Related issue

The following example did not return a result, but the interval [14, 15[ is the correct answer.

LevelCellArray<1> domain(5);
CellArray<1> ca;

domain.add_interval_back({0, 32}, {});
ca[5].add_interval_back({30, 32}, {});
ca[4].add_interval_back({14, 15}, {});

xt::xtensor_fixed<int, xt::xshape<1>> translation{1};

std::size_t level = 5;
int i             = 2;

auto boundaryCells       = difference(ca[level], translate(self(domain).on(level), -translation)).on(level);
auto translated_boundary = translate(boundaryCells, -i * translation);
auto refine_subset       = intersection(translated_boundary, ca[level - 1]).on(level - 1);

How has this been tested?

The previous example has been added in test_subset.cpp file.

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