[irm-codebase]

Fixes #85

Separates standard Modelblocks checks from module unit tests and sunsets the old test/integration_test.py file in favour of more targeted test separation.

• Moves compatibility and integration tests to tests/modelblocks/, which is now fully managed by the template.
  • convention_test.py: validates that files and behaviour specified in the Modelblocks convention.
  • module_integration_test.py: tests emulating importing the module into another workflow live here.
• Adds tests/unit/, coupled with a new pixi run test-unit task. It lets you run unit tests on workflow/scripts/*.py code.
  • new environment (unit-test) which extends the module environment, resolved in tandem with it.
  • A new CI task running these unit tests was added to .github/workflows/pr-ci.yml. It will fail by default (to force people to add at least a couple of unit tests).
  • Test code is generally shielded from template updates (only conftest.py is seen by the template, which is quite minimal).
• Additional updates to CI, documentation, and copier rules related to these changes.
• Added a short migration guidance that will show up after users run copier update.

If this is accepted, I'll submit a sister PR to the docs specifying the new testing method.

[irm-codebase]

At long last, here is the first item from the workshop!
Took a couple of iterations. I tried my best to keep the template lean.

I'd be interested in feedback from @ekatef, @fneum, @davide-f, or @lkstrp, of course!