Facilitate project contributions

[bittremieux]

• Enforce a consistent code style using black.
• Provide a git pre-commit hook for automatic code formatting.
• Include contributing guidelines.
• Include a code of conduct.
• Use setuptools_scm to automatically get the package number.
• Set up GitHub Action for automatic package release.
• Set up GitHub Action for unit tests.
• Extract notebooks to a separate repository and reorganize code directories.
• Move the documentation to readthedocs.