Add perceptually shaded slope map to terrain attributes

[rhugonnet]

Could think of adding the Perceptually-Shaded Slope Map (PSSM) as another terrain attribute for visualization: https://utppublishing.com/doi/10.3138/carto.49.4.2141

@friedrichknuth Do you know about this?

[friedrichknuth]

I did not - but this looks very interesting!