Create Pull Request template that reminds people to run the full test suite

[hugobuddel]

Occasionally we merge a PR that breaks the notebooks. The notebooks are only ran in the CI when triggered manually, because otherwise they take too long.

We should add a Pull Request template that reminds people to run the full test suite before merging / before marking the PR as ready.

See https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/creating-a-pull-request-template-for-your-repository

Something like:

• I have ran the full test suite and all tests including the notebooks pass.