Testing infrastructure

Tests are run on GitHub Actions, configured in .github/workflows/main.yml.

In addition to unit tests, now any code in the documentation that shows how to build example track hubs is handled like this:

• extract code from documentation (see ci/example_hubs.tsv for the list of files from which code is extracted)

• execute code to build track hub (see ci//build_examples.py)

• upload code and built track hub (and data, if relevant) to the trackhub-demo repository

• run hubCheck on the just-uploaded hubs (see ci/check_hubs.py)

The just-built track hubs are then live, and linked to from within the documentation.