Add/review crs for ALASKA2020 data

Add crs information to ALASKA2020 datasets, which likely have the same crs (but please check this):

• s3://mdmf/gdp/alaska_et_2020_era-interim_reanalysis.zarr
• s3://mdmf/gdp/alaska_et_2020_ccsm4_historical_simulation.zarr
• s3://mdmf/gdp/alaska_et_2020_gfdl_historical_simulation.zarr

They have the following THREDDS catalog entries, if that is helpful to have:

• https://cida.usgs.gov/thredds/catalog.html?dataset=cida.usgs.gov/alaska_et_2020/era-interim_reanalysis
• https://cida.usgs.gov/thredds/catalog.html?dataset=cida.usgs.gov/alaska_et_2020/ccsm4_historical_simulation
• https://cida.usgs.gov/thredds/catalog.html?dataset=cida.usgs.gov/alaska_et_2020/gfdl_historical_simulation