Infrastructure: Observational data pipeline and preprocessing

## Description
Set up standardized observational datasets for all Tier II evaluation.

## Datasets needed
- **Temperature**: NASA GISS, Berkeley Earth, HadCRUT, NOAA GlobalTemp, HadSST, HadISST, CRU TS
- **Precipitation**: GPCP, IMERG, MSWEP
- **Radiation**: CERES EBAF, ERBE, CERES SYN
- **Sea ice**: NSIDC Sea Ice Index, OSI SAF
- **Reanalysis**: ERA5, ECCO (for MHT validation)
- **Extended**: Argo, MODIS, CloudSat/CALIPSO, ISCCP, RSS

## Notes
- All data should be regridded to a common grid
- Post-2015 test period extraction
- Handle observational uncertainty estimates for CRPS calculation