[py] Move database and protocols in dataset.

src/bob/pipelines/__init__.py

src/bob/pipelines/dataset/__init__.py

src/bob/pipelines/dataset/database.py

src/bob/pipelines/dataset/protocols/__init__.py

src/bob/pipelines/dataset/protocols/archive.py

src/bob/pipelines/dataset/protocols/hashing.py

src/bob/pipelines/dataset/protocols/retrieve.py

tests/test_database.py