QuadTree tree = new QuadTree(0, 0, 10, 0);
assertEquals(tree.mass(), 0);
assertTrue(tree.addNode(new TestNode(1, 1)));
assertEquals(1, tree.mass());
assertEquals(1, tree.x(), eps);
assertEquals(1, tree.y(), eps);
assertTrue(tree.addNode(new TestNode(9, 9)));
assertEquals(tree.mass(), 2);
assertEquals(5, tree.x(), eps);