Download Copernicus Glo30 Elevation

A region to extract: the bbox that prunes, and the shape that clips.

region_wkt lives on the base rather than on the Overture subclass because a corridor is a corridor for elevation too. Without it the elevation download took only the bbox, so a drawn corridor from San Francisco to Los Angeles fetched its whole 6 by 5 degree bounding box - most of which the corridor never passes over - while the vector download beside it clipped correctly.