Add Arctic/REMA DEM mosaics over the Polar/Icy regions

[ShashankBice]

The Arctic DEM and Reference Elevation Model of Antarctica (REMA) have come a long way since the last mentioned issue to include these datasets (presumably in #162). The individual strips are co-registered using ICESat/CryoSat points, and mosaiced into Polar Stereographic Grids at 2m resolution. Maybe include all the updated mosaics into the next build ?

• Arctic DEM: https://www.pgc.umn.edu/data/arcticdem/
• REMA: https://www.pgc.umn.edu/data/rema/
  Feel free to ignore if this has been already done :)