Add geomorphon function

[rafaqz]

A geomorphon classifier would be nice to have here.

See:
https://svn.osgeo.org/grass/grass-addons/grass7/raster/r.geomorphon/r.geomorphon.html
https://search.r-project.org/CRAN/refmans/whitebox/html/wbt_geomorphons.html

[evetion]

Do you have a specific need/thoughts about it? I came across it earlier, but it wasn't that straightforward to implement.

[rafaqz]

Nothing direct, I'm in a workshop ecologists talking about using it as a simple terrain classifier. It seems like an interesting and alternative to the methods already here, and I just assumed it should go in this package if there was to be a Julia version. But yeah, maybe not so straightforward, I didn't actually look at the algorithms yet.

https://www.sciencedirect.com/science/article/abs/pii/S0169555X12005028?via%3Dihub