[common] Move reusable files to new common package

src/mednet/scripts/saliency/completeness.py

src/mednet/scripts/saliency/generate.py

src/mednet/scripts/train.py

src/mednet/scripts/train_analysis.py

tests/conftest.py

tests/test_cli.py

tests/test_database_split.py

tests/test_image_utils.py

tests/test_summary.py

tests/test_transforms.py