LithologiesClassesVisual3d.render_classes_planar should be able to scale the z axis

[jmp75]

On some data sets such as the Condamine where depth of bore can go really deep, the data cube may end up with more than a 1000 depths slices but less in X/Y. This ends up looking odd (really long). Is it possible to have something that is more spatially meaningful (albeit with a scaling factor still needed because in isometric view then the Z axis may be completely squashed).