fix: harmonize all the Database Interface classes.

• Ensure all Database Interface classes accept protocol as parameter.
• Make the reason for test skipping more obvious by splitting the Database creation and the Sample loading tests.

Fixes #51.

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

 """Config file for the CASIA FASD dataset.
+
 Please run ``bob config set bob.db.casia_fasd.directory /path/to/database/casia_fasd/``
-in terminal to point to the original files of the dataset on your computer."""
+in a terminal to point to the original files of the dataset on your computer.
+"""
 
 from bob.pad.face.database import CasiaFasdPadDatabase