Cleanup face cropping helpers

The available embedding extractors expect various types of cropping (various cropped image size, and more or less tight). Currently those defaults are provided through some helpers functions (https://gitlab.idiap.ch/bob/bob.bio.face/-/blob/master/bob/bio/face/config/baseline/helpers.py) that are not well documented and confusing to follow. The aim is to cleanup and document better the face cropping, mainly :

1. Baselines cleanup

• Hard code specific cropped positions for available baselines directly in the associated config file
• Set the correct annotator that should be used with each extractor (-> the same one than during training)

2. Helpers cleanup

• Refactor to provide only few, general purpose, default cropped positions (e.g. wide & tight crops)
• Document the available default crops

I'll be working on that.