Refactoring and cleanup in line with backend and pylib

examples/dataset-creation-example.sh

examples/model-training-example.sh

ocr4all_pixel_classifier_frontend/convert_colors.py

ocr4all_pixel_classifier_frontend/evaluate.py

ocr4all_pixel_classifier_frontend/find_segments.py

ocr4all_pixel_classifier_frontend/gen_masks.py

ocr4all_pixel_classifier_frontend/generate_color_map.py

ocr4all_pixel_classifier_frontend/inspect_color_map.py

ocr4all_pixel_classifier_frontend/main.py

ocr4all_pixel_classifier_frontend/migrate_model.py

ocr4all_pixel_classifier_frontend/predict.py

ocr4all_pixel_classifier_frontend/train.py

requirements.in