V18 seaice observations

[guillaumevernieres]

In order of priority:

• VIIRS seaice thickness and temp
• IceSat and CryoSat freeboard (https://nsidc.org/data/atl10ql/versions/6#anchor-data-access-tools)
• SAR
• seaice drift (interesting for coupled covariance and verification)
• IMS for verification (low priority)