Preprocessor does not use the load method of the BioFile class

Created by: 183amir

The databases provide a load method in its File class but this is not used in the preprocessor when data is read. read_original_data: https://github.com/bioidiap/bob.bio.base/blob/master/bob/bio/base/preprocessor/Preprocessor.py#L87

Some databases may contain more than one sample in one file (like videos and audios with two channles); while I understand that this is handled in bob.bio.video for video files, it is not clear how this can be handled for audio files with two channels in them.

If the preprocessor was calling the load method of the File (BioFile) class, we could use logical paths for File.path instead of the actual path and handle this in the load method. For example, File.path would be origpath_A or origpath_B depending on the channel and then the load method would return only channel A or B depending on the logical path that was requested.