Apply Part-of-Speech (POS) tagging on each token.
        Uses the default NLTK tagger if no language-specific tagger could be loaded (English is assumed then as
        language). The default NLTK tagger uses Penn Treebank tagset
        (https://ling.upenn.edu/courses/Fall_2003/ling001/penn_treebank_pos.html).
        The default German tagger based on TIGER corpus uses the STTS tagset
        (http://www.ims.uni-stuttgart.de/forschung/ressourcen/lexika/TagSets/stts-table.html).

def pos_tag(self):
        """
        Apply Part-of-Speech (POS) tagging on each token.
        Uses the default NLTK tagger if no language-specific tagger could be loaded (English is assumed then as
        language). The default NLTK tagger uses Penn Treebank tagset
        (https://ling.upenn.edu/courses/Fall_2003/ling001/penn_treebank_pos.html).
        The default German tagger based on TIGER corpus uses the STTS tagset
        (http://www.ims.uni-stuttgart.de/forschung/ressourcen/lexika/TagSets/stts-table.html).
        """
        self._require_tokens()
        self._require_no_ngrams_as_tokens()

        self._invalidate_workers_tokens()
        logger.info('POS tagging tokens')
        self._send_task_to_workers('pos_tag')
        self.pos_tagged = True

        return self