ZT files are processed even when no ZT processing is wanted

When a ZT database is instantiated, but used without ZT normalization enabled, the files for ZT norm are still processed. This is annoying.

A fix would include changing the basic database interface of the all_files function: https://gitlab.idiap.ch/bob/bob.bio.base/blob/master/bob/bio/base/database/database.py#L370 and https://gitlab.idiap.ch/bob/bob.bio.base/blob/master/bob/bio/base/database/database.py#L640 The fix would introduce a new parameter, which tells if ZT score files should be included or not. When called from the FileSelector, e.g.: https://gitlab.idiap.ch/bob/bob.bio.base/blob/master/bob/bio/base/tools/FileSelector.py#L125 The information need to be provided. How this is done, I need to investigate.