add year week as coord not replacing forecast_time

Closes https://renkulab.io/gitlab/aaron.spring/s2s-ai-challenge/-/issues/29

• conservative regridding
• fixes longitude=0 NaNs
• add CHANGELOG
• check files on renku
• ensure notebooks still work
• rm weekofyear coord from obs_2000_2019 / or just upload zarr again manually
• release afterwards