Adding support for importing/exporting confidence in YOLO formats

[Rusteam]

What changes are proposed in this pull request?

YOLOv4 and YOLOv5 import/export with confidence. Issue

How is this patch tested? If it is not, please explain why.

Assert equal detection confidence bounds between base and after export/import.
Updated tests in tests/unittests/import_export_tests.py to handle default include_confidence=False
and non-default include_confidence=True cases in test_yolov4_dataset and test_yolov5_dataset

Release Notes

Is this a user-facing change that should be mentioned in the release notes?

• No. You can skip the rest of this section.
• Yes. Give a description of this change to be included in the release
  notes for FiftyOne users.

Set include_confidence=False when exporting a dataset in YOLOv4/YOLOv5 formats to include confidence.
Importing datasets in YOLOv4/YOLOv5 formats automatically loads confidence if it's present in a txt file with detections.

What areas of FiftyOne does this PR affect?

• App: FiftyOne application changes
• Build: Build and test infrastructure changes
• Core: Core fiftyone Python library changes
• Documentation: FiftyOne documentation changes
• Other

[Rusteam]

any updates on this one?

[brimoor]

LGTM. Thanks for adding this feature @Rusteam!

And special thanks for including the unit tests 💯

[Rusteam]

thanks for merging it!