Optimize cells_intersect and add find_adjacent_cells_related routine
Improves performance of cells_intersect by implementing Gilbert–Johnson–Keerthi distance algorithm (see https://en.wikipedia.org/wiki/Gilbert-Johnson-Keerthi_distance_algorithm).
Improves performance of cells_intersect by implementing Gilbert–Johnson–Keerthi distance algorithm (see https://en.wikipedia.org/wiki/Gilbert-Johnson-Keerthi_distance_algorithm).