The parameter `protocol` on method `model_ids` in BioDatabase is never used by `bob.bio.base`

In the current design, the implementer of a BioDatabase derivative needs to implement a method called model_ids which contains an attributed called protocol. However, this method is never called with such parameter as bob.bio.base relies on the FileSelector that does not know about protocols.

In short, the BioDatabase object needs to have a self.protocol previously set and use that inside model_ids() instead of the passed value.

The API of that method in BioDatabase needs revision and all existing implementations.