Post discretization

[JulianGeis]

• Realted PR with description: Post-Discretization of network infrastructure pypsa-ariadne#62

Closes # (if applicable).

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). -> no tests
• [] 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.

[fneum]

Thanks @JulianGeis. I added a unit test.