Refactor entities extraction with lxml

dan/datasets/extract/__init__.py

dan/datasets/extract/arkindex.py

dan/datasets/extract/db.py

dan/datasets/extract/exceptions.py

dan/datasets/extract/utils.py

docs/css/ner.css

docs/usage/datasets/extract.md

mkdocs.yml

requirements.txt

tests/conftest.py

tests/data/entities.yml

tests/data/tokens/end_tokens.yml

tests/data/tokens/no_end_tokens.yml

tests/test_db.py

tests/test_extract.py