message: Add unit test for squared_distance between two BoundingBoxes.
diff: #include "axom/primal/operators/squared_distance.hpp"
  #include "axom/primal/geometry/Triangle.hpp"
  #include "axom/primal/geometry/Segment.hpp"
  +#include "axom/primal/geometry/Vector.hpp"
  using namespace axom;
  @@ -167,3 +168,27 @@ TEST(primal_squared_distance, point_to_segment)
  dist = primal::squared_distance(q2, S);
  EXPECT_DOUBLE_EQ(primal::squared_distance(q2, A), dist);
  }
  +
  +//------------------------------------------------------------------------------
  +TEST(primal_squared_distance, bbox_to_bbox)
  +{
  + constexpr int DIM = 3;
  + using CoordType = double;
  + using QPoint = primal::Point<CoordType, DIM>;
  + using QVector = primal::Vector<CoordType, DIM>;
  + using QBBox = primal::BoundingBox<CoordType, DIM>;
  +
  + const QBBox middle(QPoint {0.0, 0.0, 0.0}, QPoint {1.0, 1.0, 1.0});
  + const QBBox north(QPoint {0., 10., 0.}, QPoint {1., 11., 1.});
  + const QBBox east(QPoint {10., 0., 0.}, QPoint {11., 1., 1.});
  + const QBBox northeast(QPoint {10., 10., 0.}, QPoint {11., 11., 1.});
  + const QBBox northeastup(QPoint {10., 10., 10.}, QPoint {11., 11., 11.});
  + QBBox touching(middle);
  + touching.shift(QVector::make_vector(0.9, 0.99, 0.999));
  +
  + EXPECT_DOUBLE_EQ(squared_distance(middle, north), 81.);
  + EXPECT_DOUBLE_EQ(squared_distance(middle, east), 81.);
  + EXPECT_DOUBLE_EQ(squared_distance(middle, northeast), 162.);
  + EXPECT_DOUBLE_EQ(squared_distance(middle, northeastup), 243.);
  + EXPECT_DOUBLE_EQ(squared_distance(middle, touching), 0.);
  +}
  