Include validation for sector networks

[danielelerede-oet]

Changes proposed in this Pull Request

This PR extends validation of electricity generation, capacity and demand to support sector-coupled networks (especially considering the conversion of conventional generators to links). Those links are identified from their bus connections, while the existing behaviour for electricity-only networks is preserved.

Checklist

• I consent to the release of this PR's code under the AGPLv3 license and non-code contributions under CC0-1.0 and CC-BY-4.0.
• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented, including updates to docstrings for meaningful functions.
• Newly introduced dependencies are added to envs/environment.yaml.
• Changes in configuration options are added in all of config.yaml.
• A note for the release notes doc/release-notes.md is amended in the format of previous release notes, including reference to the requested PR.