code: public Triangle findCellTriangleOf(Point3D point) {
          int x_index = (int) ((point.x - indexRegion.minX()) / x_size);
          int y_index = (int) ((point.y - indexRegion.minY()) / y_size);
          return grid[x_index][y_index];
      }
docs: Finds a triangle near the given point
  
  @param point a query point
  @return a triangle at the same cell of the point