Classy-classification

Added support for ONNX enhanced zer-shot support.

closes: 27

There was some unexpected behaviour with labels being sorted incorrectly, leading to wrong predictions.

closes 36
closes 28
closes 38

sub-optimal config
`n_jobs` parallelization issues

- no vector bug when `doc.has_vector` resulted in an error instead of creating an empty vector 22
- onnx extras was not properly added to the `pyproject.toml`as optional dependency
- added parallel processing for fitting the SVC using gridsearch