Thingsvision

- major `README` overhaul
- enabled `thingsvision.__version__` for automatic versioning

- THINGSvision changed to `thingsvision`. This is more intuitive.
- We've added a low-memory option for feature extraction.
- `extractor.extract_features()` now takes extra optional arguments `output_dir` and `steps`.
- if `output_dir` is defined, features will be written to disk every `steps` mini-batches (default: 100).
- iteration across mini-batches got abstracted into the `base` extractor class.

- modularized feature extraction (added more abstractions)
- added base extractor class from which all source-specific extractor classes are supposed to inherit
- mixin classes for both `PyTorch` and `TensorFlow`
- source-specific extractor classes that inherit from both the base extractor class and mixin classes like so

python
dataclass(repr=True)
class TimmExtractor(BaseExtractor, PyTorchMixin):
...


- modified unittests according to most recent changes (not much has change here)