Fix simplify network to 380kV

[davide-f]

Closes #314

Changes proposed in this Pull Request

This PR aims at solving the bug described into #314.
The circuits number is changed proportionally to the ratio between i_nom and v_nom of the line types.

Checklist

• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented.
• Newly introduced dependencies are added to envs/environment.yaml and envs/environment.docs.yaml.
• Changes in configuration options are added in all of config.default.yaml, config.tutorial.yaml, and test/config.test1.yaml.
• Changes in configuration options are also documented in doc/configtables/*.csv and line references are adjusted in doc/configuration.rst and doc/tutorial.rst.
• A note for the release notes doc/release_notes.rst is amended in the format of previous release notes.

[pz-max]

A test was performed here: pypsa-meets-earth/pypsa-earth#443

[pz-max]

@FabianHofmann and @martacki .. just FYI in case you started working on it.
Davide found possibly a solution to the bug you were assigned to #314.
The test looks promising (see link shared above).

Could one be assigned to review/test this PR?