Add ALOS World 3D 30m as slope source

[planemad]

Is your feature request related to a problem? Please describe.

The hill slope layer is currently generated from two different sources: ISRO/CartoDem and NASA/Aster. Both mostly match but have have slight differences. Adding one more independent source will add confidence to the existing datasets.

Describe the solution you'd like

Add hill slopes generated from Japan Aerospace Exploration Agency (JAXA)

• Homepage: https://www.eorc.jaxa.jp/ALOS/en/dataset/aw3d30/aw3d30_e.htm
• Download: https://portal.opentopography.org/raster?opentopoID=OTALOS.112016.4326.2
• Terms: (CC-BY) https://earth.jaxa.jp/en/data/policy/