feat: add store_model option to save linopy model after solving

[FabianHofmann]

Add solving.options.store_model config option that stores the linopy model to a NetCDF file in results/models/ after solving. Includes validation to prevent use with rolling_horizon mode.

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.

[fneum]

Just one question about output_model function.

[fneum]

RuleException in file "/home/runner/work/pypsa-eur/pypsa-eur/rules/solve_electricity.smk", line 6:
Only input files can be specified as functions