Extend 1s and 3s coverage via REMA and ArcticDEM

[PaulWessel]

FYI: From a tip by Brook Tozer, I am downloading REMA and ArcticDEM. These are 8 meter pixel resolutions of DEMs, and the plan would be to filter these to 1s and 3s and add a bunch of new "SRTM" tiles beyond the ±60 latitude limitation. We are talking 1 Tb of data per dataset here so probably not something I can do in time for 6.3, but it would just be an eventual update to earth_relief. The wget command is chugging away as we speak. I can see my decision to get the latest MacBook M1 Max with 8 Tb SSD will be full soon enough...

[weiji14]

REMAv2 DEM is now out as well. The mosaics now go down to 2 meters (!!) compared to 8 meters before. You can get it at https://data.pgc.umn.edu/elev/dem/setsm/REMA/mosaic/v2.0/2m/ or via AWS on https://registry.opendata.aws/pgc-rema. Should be about 8.4 TB in total 🙈

Oh, and there should be a new version of the ArcticDEM mosaics too (version 4, probably early next year), so maybe wait until that gets released rather than processing them now.