Implement roughness

[wkearn]

GRIDobj.roughness provides a few different local terrain roughness metrics, which are implemented using various filtering methods. I am not sure these all need to be implemented in libtopotoolbox, but they depend on the filtering primitives which we do want to implement.

• Terrain ruggedness index: local standard deviation
• Topographic position index: mean filtering (Implement filter #110)
• Roughness: maximum/minimum filtering (Implement dilate #105, Implement erode #108). This could be more efficiently computed
• Ruggedness: surface normals and mean filtering (Implement filter #110)