Re-estructured Preprocessor to simplify experimentation and configuration

bob/bio/vein/configurations/maximum_curvature.py

bob/bio/vein/configurations/repeated_line_tracking.py

bob/bio/vein/configurations/wide_line_detector.py

bob/bio/vein/database/__init__.py

bob/bio/vein/database/fv3d.py

bob/bio/vein/database/utfvp.py

bob/bio/vein/database/verafinger.py

bob/bio/vein/preprocessor/__init__.py

bob/bio/vein/preprocessor/mask.py

bob/bio/vein/preprocessor/normalize.py

bob/bio/vein/preprocessor/preprocessor.py