madd oil generator to bus EU oil

[martacki]

Changes proposed in this Pull Request

Add a oil generator to bus EU oil, when adding the transport sector. Without it, existing loads lead to a LHS RHS imbalance in the LP file and the isolated transport sector model cannot be solved (Error: ValueError: Empty LHS with non-zero RHS in nodal balance constraint).

Checklist

• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented.
• Changed dependencies are added to envs/environment.yaml.
• Changes in configuration options are added in all of config.default.yaml.
• Changes in configuration options are also documented in doc/configtables/*.csv.
• A release note doc/release_notes.rst is added.

[fneum]

Thanks! We can directly use add_carrier_buses for this.