GLORYS utility function

[NoraLoose]

This PR introduces a GLORYS utility function and updates dataset documentation.

Key Changes

1. New Utility Function

• Adds a GLORYS utility function as proposed in Utility function to determine GLORYS download bounds from a ROMS grid #443.

2. Dataset Documentation Updates (datasets.ipynb)

• Shows how to download external datasets:
  • GLORYS via copernicusmarine
    • Without the utility function for global datasets
    • With the utility function for regional datasets
  • Unified BGC data from our Google Drive

Internal Refactors

To support the new utility function, the Dataset class was refactored, moving some methods to standalone functions for broader usability:

• concatenate_longitudes
• choose_subdomain
• get_target_coords (now accepts a grid_ds instead of a full grid)

Checklist

• Closes Utility function to determine GLORYS download bounds from a ROMS grid #443
• Tests added
• Passes pre-commit run --all-files
• Changes are documented in docs/releases.md
• New functions/methods are listed in docs/api.rst
• New functionality has documentation

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[NoraLoose]

Closing this because I re-opened the same PR #457 on the main rather than my fork, so that the copernicusmarine credentials actually work via secrets.