Changes proposed in this Pull Request

Separates functions add_aviation() and add_shipping() from add_industry().

This restructuring will simplify endogenizing the shipping and aviation sectors.

No other changes.

Checklist

• I tested my contribution locally and it works as intended.
• Code and workflow changes are sufficiently documented.
• Changed dependencies are added to envs/environment.yaml.
• Changes in configuration options are added in config/config.default.yaml.
• Changes in configuration options are documented in doc/configtables/*.csv.
• Sources of newly added data are documented in doc/data_sources.rst.
• A release note doc/release_notes.rst is added.