Add pre-commit hook for Black formatting

[andodet]

Summary

Thanks for the interesting package, I can see it softening the burden of writing lots of boiler-plate code for standard-ish modeling tasks.

I've noticed code formatting with black is enforced in CONTRIBUTING.md. Might be a good idea to add a pre-commit hook to take that out of the equation when reviewing pull requests.

Details

Checklist

• all tests are passing (see README.md on how to run tests)
• if you created a new template: it contains a file test-inputs.yml, which specifies a few input values to test the code template (the test is then automatically run by pytest)
• you formatted all code with black
• you checked all new functionality live, i.e. in the running web app
• any generated code is formatted nicely, both in .py and in .ipynb ("nicely" = comparable to the existing templates)
• you added comments in your code that explain what it does
• the PR explains in detail what's new