fix: harmonize all the database interface classes.

src/bob/pad/face/config/casia_fasd.py

src/bob/pad/face/database/casia_fasd.py

src/bob/pad/face/database/casia_surf.py

src/bob/pad/face/database/mask_attack.py

src/bob/pad/face/database/oulu_npu.py

src/bob/pad/face/database/replay_attack.py

src/bob/pad/face/database/replay_mobile.py

src/bob/pad/face/database/swan.py