Refactor base extractors
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This PR:
- Refactors base extractors as subclasses of `BaseExtractor`:
  - this is an abstract class defining the interface with:
    - `is_extractable`: abstract class method
    - `extract`: abstract static method
- Implements abstract `MagicNumberBaseExtractor` (as subclass of `BaseExtractor`): 
  - this has a default implementation of `is_extractable`
  - this improves performance (reducing the number of file reads) by allowing passing already read `magic_number`
- Refactors `Extractor`:
  - reads magic number from file only once

This PR deprecates:
```python
is_extractable, extractor = self.extractor.is_extractable(input_path, return_extractor=True)
self.extractor.extract(input_path, output_path, extractor=extractor)
```
and uses more Pythonic instead:
```python
extractor_format = self.extractor.infer_extractor_format(input_path)
self.extractor.extract(input_path, output_path, extractor_format)
```