Fixing sphinx warnings

bob/bio/base/algorithm/Algorithm.py

bob/bio/base/algorithm/BIC.py

bob/bio/base/database/database.py

bob/bio/base/database/file.py

bob/bio/base/extractor/Extractor.py

bob/bio/base/extractor/Linearize.py

bob/bio/base/preprocessor/Preprocessor.py

bob/bio/base/tools/FileSelector.py

bob/bio/base/tools/command_line.py

bob/bio/base/utils/singleton.py

doc/conf.py

doc/extra-intersphinx.txt

doc/implementation.rst

doc/implemented.rst

doc/installation.rst

requirements.txt