Allow allow subregional NUTS selection.

[bobbyxng]

Changes proposed in this Pull Request

• Extension of NUTS administrative clustering
• Allows a more fine-grained resolution with NUTS regions, e.g. "DEA" at level 3, while the neighbouring NUTS regions within DE are level 1 or 2 for example

Checklist

Required:

• Changes are tested locally and behave as expected.
• Code and workflow changes are documented.
• A release note entry is added to doc/release_notes.rst.

If applicable:

• Changes in configuration options are reflected in scripts/lib/validation.
• For new data sources or versions, these instructions have been followed.
• New rules are documented in the appropriate doc/*.rst files.