Extend 1s and 3s coverage via REMA and ArcticDEM

FYI: From a tip by Brook Tozer, I am downloading [REMA](https://www.pgc.umn.edu/data/rema/) and [ArcticDEM](https://www.pgc.umn.edu/data/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...