Add parcels._datasets.remote

[VeckoTheGecko]

Description

This PR adds tooling for ingesting remote datasets that are:

• For different purposes
  • testing or tutorials
• Backed by netcdf files or (the new approach) backed by a zipped Zarr store
  • This requires minimal changes to the original data repo (except adding a new folder data-zarr for when we add zipped zarr files

This PR also:

• Removes the download_example_dataset util (and renames the list_example_dataset util)
• Removes the data downloading utils from the root parcels namespace
• Creates a parcels._datasets.remote module as well as a thin wrapper module parcels.tutorial
• Adds a function parcels._datasets.remote.open_dataset which returns an xarray dataset
• So that CI passes, updates all references across the codebase (except references in v3 folders that are yet to be migrated)

Checklist

• Closes Update tutorial dataset storage and loading #2571
• Tests added
• This PR targets the correct branch (main for normal development, v3-support for v3 support)

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.
  • Describe how you used it (e.g., by pasting your prompt): Writing docstrings and updating examples to use new ingestion method

[VeckoTheGecko]

I thought that this approach was better than migrating the datasets to Zarr (i.e., working closer to the source netcdf files, without having to think about deprecation cycles and data storage etc.)

[erikvansebille]

Looks good; a few small comments below