Add new line type for NEP

[p-glaum]

This PR adds a new line type in pypsa. This is needed, because many line projects in NEP are upgraded to those new line types, which can carry higher currents (4kA) for each circuit. This leads to an effective power transfer of ~2600MW per circuit.

Changes proposed in this Pull Request

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• Newly introduced dependencies are added to environment.yaml, environment_docs.yaml and setup.py (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.