fix p_nom aggregation, and model line as link per default

[pz-max]

Closes #431

Changes proposed in this Pull Request

• Change p_nom: min to p_nom: sum. This guarantees that no capacity will be lost by the aggregation/clustering process.
• tutorial and default config use hvdc_as_line: false, the config.test as true

Checklist

• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented.
• Changes in configuration options are added in all of config.default.yaml, config.tutorial.yaml, and test/config.test1.yaml.
• 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.