port to imageio-ffmpeg and code cleanup

bob/pad/face/database/__init__.py

bob/pad/face/database/database.py

bob/pad/face/database/replay_mobile.py

bob/pad/face/test/test_databases.py

bob/pad/face/test/test_utils.py

bob/pad/face/utils/load_utils.py

conda/meta.yaml

develop.cfg

doc/api.rst

requirements.txt