Fix simplify network overestimation

[davide-f]

Closes #283

Changes proposed in this Pull Request

This PR aims at fixing the problem of the simplify network approach that overestimates the line capacity, and quite consistently.

With the proposed fix, that issue is solved: the calculation of the number of circuits is adapted according to the ratio of voltages and nominal capacities of the lines.

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, including reference to the requested PR.

[davide-f]

Before merging this, I'd like to have the opinion by the PyPSA-Eur group.
I'll open a PR also in PyPSA-Eur soon

[pz-max]

The comparison suggests that the changes successfully fix the issue