Changes proposed in this Pull Request

This PR proposes to use the AC buses from the PyPSA network directly to define the nodes for the low-voltage buses instead of the population layout index. This is intended to make the implementation more robust to create a low-voltage bus for each AC bus.

Checklist

• I tested my contribution locally and it works as intended.
• Code and workflow changes are sufficiently documented.
• Changed dependencies are added to envs/environment.yaml.
• Changes in configuration options are added in config/config.default.yaml.
• Changes in configuration options are documented in doc/configtables/*.csv.
• Sources of newly added data are documented in doc/data_sources.rst.
• A release note doc/release_notes.rst is added.