Smartpipeline

- Mypy checked

- Logging now works properly
- Tests coverage
- Refactoring, also of class/method names

- retry policy on stage failures on items
- `on_end` method on stages called at pipeline termination
- documentation fixes

- critical bug fixes on concurrency
- documentation on multiprocessing
- an example script
- BREAKING CHANGES:
- `use_threads` parameter has been substituted with `parallel`, values are inverted
- item timings are now in seconds
- `get_exception` of an `Error` without associated Exception now returns `None`
- `Error` class is now called `SoftError`, `Error` becomes the general base class
- `build()` must be used at the end of the methods chain in `Pipeline` construction

- Heavy refactoring
- Documentation
- Pipeline queues size
- Critical fixes on concurrency

- Batch stages