Wtpsplit

- Fix a bug affecting some hash embeddings of the `canine-*` models which reduced accuracy (please upgrade to this version!).
- Add a guide on adapting to your custom data: https://github.com/bminixhofer/wtpsplit#advanced-usage.

- fix error with text where length is not a multiple of 4 and shorter than 512 characters in `canine-s-*` models (98).

- add `strip_whitespace` flag.
- fix bug with some zero-length sentences being returned if there is lots of trailing whitespace.

- fix argument propagation from model wrapper (95 97)

- Speed up pre- & postprocessing via better vectorization (94).
- Proper onnxruntime support for the `wtp-bert-*` models, although onnx models are currently not much faster (or even slower) than PyTorch models for some reason. Will continue to look into that.
- Adds missing `pandas` requirement (fixing 92).
- Lower bounds on `transformers` and other requirements to make sure all the functionality we need is there.
- Removes `torch` from requirements since users will want to install it themselves depending on their hardware setup, and it's not required anymore when using only the onnx models.

- Added missing `get_threshold` function
- `wtp.split` adapted to some style now also allows changing the threshold via `wtp.split(..., threshold=threshold)`. Was previously overwritten by the default.