Add protocol input argument to probe_file_sets function

This is better to add protocol as an input argument to probe_file_sets function. Something like:

    def probe_file_sets(self, model_id=None, group='dev', protocol=None):
        """probe_file_sets(model_id = None, group = 'dev') -> files

        Returns a list of probe FileSet objects, respecting the selected or current protocol.
        If a ``model_id`` is specified, only the probe files that should be compared with the given model id are returned (for most databases, these are all probe files of the given group).
        Otherwise, all probe files of the given group are returned.

        **Parameters:**

        model_id : int or str or ``None``
          A unique ID that identifies the model.

        group : one of ``('dev', 'eval')``
          The group to get the probe files for.

        protocol : str or ``None``
          The protocol to get the probe files for. 

        **Returns:**

        files : [:py:class:`bob.bio.base.database.BioFileSet`] or something similar
          The list of file sets used to probe the model with the given model id."""
        protocol = protocol or self.protocol
        if model_id is not None:
            file_sets = self.object_sets(protocol=protocol, groups=group, model_ids=(model_id,), purposes='probe',
                                         **self.all_files_options)
        else:
            file_sets = self.object_sets(protocol=protocol, groups=group, purposes='probe',
                                         **self.all_files_options)
        return self.sort(file_sets)