New Features:
- added docstrings and auto-generated documentation. (see `/docs`)
- further internal changes:
- added a `pyproject.toml` configuration file
- added `poetry` for dependency management and publishing
- added Python formatting (`black`) and documentation checking to CI
- updated README to include more examples
Breaking Changes:
- added requirement for Python 3.6+
- refactored all `NML`, `Node`, `Tree`, `Comment`, ... objects to inherit directly from `namedtuple`