Add embedded example dataset and -test-run workflow

[jravani]

Summary

Add a small embedded example dataset and a -test-run workflow, so users can quickly try City2TABULA without preparing their own input data.

Motivation

At the moment, first-time setup can feel heavy because users need to prepare datasets, configure the environment, and understand the workflow before they can verify that the tool is working.

Providing a built-in example run would improve onboarding and also make pipeline testing more reproducible.

Proposed improvement

Introduce a small example dataset bundled with the project, together with a dedicated -test-run flag or equivalent command that:

• loads example CityGML and/or CityJSON input data
• runs the required pipeline steps automatically
• demonstrates the expected workflow with minimal setup
• helps users understand the database structure and processing flow
• can be reused in automated or manual testing

Expected benefits

• easier onboarding for new users
• faster verification that the tool is installed correctly
• more reproducible testing of the pipeline
• simpler bug reporting using a known example case
• better developer confidence when changing the workflow

Possible tasks

• define a small example dataset suitable for inclusion in the repository
• add support for a -test-run flag or equivalent command
• automate the required pipeline steps for the example run
• document the example workflow in the setup guide
• reuse the example dataset in testing where appropriate

Acceptance criteria

• a new user can run City2TABULA on bundled example data with minimal setup
• the example workflow is documented clearly
• the example run completes successfully on a clean setup
• the example workflow is suitable for reuse in testing