Add CHELSA Climatologies

[bartromgens]

https://chelsa-climate.org/downloads/

• bio/ – Bioclimatic variables (derived temperature & precipitation indices)
• clt/ – Cloud cover (cloud area fraction)
• cmi/ – Climate Moisture Index
• hurs/ – Near-surface relative humidity
• pet/ – Potential evapotranspiration
• pr/ – Precipitation
• rsds/ – Surface downwelling shortwave radiation (solar radiation)
• sfcWind/ – Near-surface wind speed
• tas/ – Mean near-surface air temperature
• tasmax/ – Maximum near-surface air temperature
• tasmin/ – Minimum near-surface air temperature
• vpd/ – Vapour pressure deficit