External Bounding Box has negative height

[tazdevil78]

call to Hexagon.getExternalBoundingBox() returns a negative value for height.

``@Test
public void negativeHeight() {
HexagonalGridBuilder builder = new HexagonalGridBuilder()
.setGridHeight(3)
.setGridWidth(3)
.setGridLayout(HexagonalGridLayout.HEXAGONAL)
.setOrientation(HexagonOrientation.FLAT_TOP)
.setRadius(138);
HexagonalGrid grid = builder.build();
Hexagon hex = (Hexagon) grid.getHexagons().iterator().next();
Rectangle rect = hex.getExternalBoundingBox();
assertTrue(rect.getHeight()>0); // [207.0, 358.5, 276.0 ,-239.0]
}```

[tazdevil78]

height for a flat top hex with a radius of 138 is sqrt(3)*138 = 239. Appears the magnitude is correct, just need to take the abs().