Pystream-pipeline

- [x] Output reading in parallel thread pipeline.
- [x] Option to use module global ThreadPoolExecutor for functional operation.
- [x] Add data argument to functional pipeline.
- [x] Pipeline is None after `Pipeline`'s `cleanup` method is called.
- [x] Serial pipeline data become None if it has been read once.

- [x] Poetry initialization
- [x] Basic staged pipeline operations.
- [x] Basic functional pipeline operations.
- [x] Usage example script -> `demo_pipeline.py`.
- [x] First documentations:
- [x] Docstrings for classes, methods, and functions.
- [x] Minimum `README.md`.
- [x] Documentation in `docs` about how `pystream` works.
- [x] Documentation in `docs` about basic API.

On-going