Backfill DWD ICON-EU

[JackKelly]

Sub-tasks:

• Copy Dexter Energy's ICON-EU archive to dynamical.org's Source Co-Op object storage bucket (Dexter's archive is a 3-year archive of a subset of ICON-EU gribs. Just 1 init per day. See this comment for more details.)
• PR to create 1-init-per-day ICON-EU Zarr from Dexter's archive of ICON-EU gribs on dynamical.org's Source Co-Op bucket. This will probably be a separate Zarr to the live-updating ICON-EU Zarr.

Related:

• DWD ICON-EU forecast task list & general discussion (for live-updating Zarr) #173

Note: I may not have time to do these tasks