GitHub Actions pipeline for JSON example file validation

[jonssonchristian]

Whist working on another PR (#243) I was thinking it would be useful to have an automated check that all of the JSON example files (under 'demo_data') validate successfully against the JSON Schema. That would save running that check manually and ensure it is never missed. I set up something similar for another project and would be happy to have a go at implementing it for this repo if you think it is a good idea.

My idea would be to write the test in Python with the pytest framework and then have a GitHub Actions pipeline that runs the test on every new PR.