Heuristically split existing solar capacities 50:50 between rootop and utility scale

[lindnemi]

Differences in the myopic test case are minor and occur at only one bus, but are aligned with the purpose of this change.

Before:

After:

2030 values change, but 2050 values are little affected

Checklist

• I tested my contribution locally and it works as intended.
• Code and workflow changes are sufficiently documented.
• Changed dependencies are added to pixi.toml (using pixi add <dependency-name>).
• Changes in configuration options are added in config/config.default.yaml.
• Changes in configuration options are documented in doc/configtables/*.csv.
• For new data sources or versions, these instructions have been followed.
• A release note doc/release_notes.rst is added.

[lindnemi]

50:50 is a heuristic value. Since rooftop share between countries differs largely, a spatially resolved split would be more accurate, but there is no data.

For the whole of the EU the current rooftop:utility split is probably 60:40 (according to https://www.solarpowereurope.org/insights/outlooks/eu-solar-market-outlook-2025-2030/detail)

I chose 50:50 because some countries may be well below 60:40 and since rooftop installations are declining. But perhaps 60:40 would be better