Spacy-layout

- Improve bounding box calculation for bottom left origin.

- Fix bounding boxes for bottom left origin.

- Add `spaCyLayout.pipe` to process multiple documents.
- Use `nlp.pipe` internally for tokenization.
- Also accept `bytes` as input.

- Add `Span.id` with running index of layout span.
- Add `Span._.heading` that returns closest heading for given span.