timit database interface is not tested and does not work

As indicated in bob.bio.base#52 (comment 11265), the timit database interface will most probably not work, as it does not implement the BioDatabase interface, but uses the raw bob.db.bio_filelist database, see: https://gitlab.idiap.ch/bob/bob.bio.spear/blob/master/bob/bio/spear/config/database/timit.py#L8

• Implement a proper interface for the timit database. I will provide a base class implementation for a BioFileListDatabase in bob.bio.base, which we can use here
• Implement a test case for the database