Initialize the project

- describe how a local develop environment can be set up
- we use poetry to manage the project
  => add pyproject.toml and poetry.lock files
- add a package for the source code
  => "src" layout structure to ensure that pytest runs the tests
     against a packaged version installed in a virtual environment
     and not the *.py files in the project directory
     (Source: https://hynek.me/articles/testing-packaging/)
- ignore poetry's artifacts in git