Use multithreading to download images at the end, remove cache as we now...

dan/datasets/extract/__init__.py

dan/datasets/extract/exceptions.py

dan/datasets/extract/extract.py

dan/datasets/extract/utils.py

docs/usage/datasets/extract.md

tests/conftest.py

tests/data/extraction/images/double_page/test-page_1.jpg

tests/data/extraction/images/double_page/test-page_2.jpg

tests/data/extraction/images/double_page/train-page_1.jpg

tests/data/extraction/images/double_page/train-page_2.jpg

tests/data/extraction/images/double_page/val-page_1.jpg

.../extraction/images/lines/text_line_test-page_1-line_1.jpg

.../extraction/images/lines/text_line_test-page_1-line_2.jpg

.../extraction/images/lines/text_line_test-page_1-line_3.jpg

.../extraction/images/lines/text_line_test-page_2-line_1.jpg

.../extraction/images/lines/text_line_test-page_2-line_2.jpg

.../extraction/images/lines/text_line_test-page_2-line_3.jpg

...extraction/images/lines/text_line_train-page_1-line_1.jpg

...extraction/images/lines/text_line_train-page_1-line_2.jpg

...extraction/images/lines/text_line_train-page_1-line_3.jpg