Port EOOffshore CMEMS ASCAT REP and NRT recipes to use beam-refactor branch

[derekocallaghan]

Following approach in #445, this will port the CMEMS ASCAT REP and NRT recipes to use the beam-refactor branch. It will involve writing at least one custom PTransform for required Dataset preprocessing.

[derekocallaghan]

It looks like two PTransform classes will be required:

• Similar to the CCMP recipe, the first transform will preprocess each Dataset, including replacing the provided time coordinates.
  • For now, the preprocess function can be wrapped with pangeo_forge_recipes.transforms._add_keys().
• The second transform will sort the PCollection using the updated time coordinates. I still need to confirm the best approach here, but it's likely that it will update the Position in each Index, something similar to what currently occurs in pangeo_forge_recipes.transforms.IndexItems.

[derekocallaghan]

This might be a candidate for the proposed separate issue for migrating tutorials from notebooks -> scripts (with CI testing)