solve_network: move extra_functionality to args

[martacki]

Closes # (if applicable).

Changes proposed in this Pull Request

Currently if solve_network is used outside pypsa-eur (for example via import), no additional extra_functionality can be passed as kwarg. If it is removed from the function and only passed as kwarg instead, this problem is resolved.

associated error: TypeError: pypsa.linopf.network_lopf() got multiple values for keyword argument 'extra_functionality'

Thoughts?

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.