Initial commit

- add the following files:
  + .gitignore => ignore poetry's and pyenv's artifacts
  + LICENSE.txt => MIT license as the project is scientific research
  + README.md => rough description of the project
  + poetry.lock
  + pyproject.toml
  + src/urban_meal_delivery/__init__.py => source code package
- use a "src" layout structure:
  + ensure that pytest runs the tests against a packaged and pip
    installed version of the source code, not the *.py files in the
    project directory
  + more info: https://hynek.me/articles/testing-packaging/