'Tag a sentence using Python CRFSuite Tagger. NB before using this function, user should specify the mode_file either by
- Train a new model using ``train\'\' function
- Use the pre-trained model which is set via ``set_model_file\'\' function
:params tokens : list of tokens needed to tag.
:type tokens : list(str)
:return : list of tagged tokens.
:rtype : list (tuple(str,str))'

def tag(self, tokens):

    return self.tag_sents([tokens])[0]
