code: def __add_tier(self, tier, token_tier_name):
          
          if tier.attrib['category'] == token_tier_name:
              self.__add_tokens(tier)
          else:
              if self.is_token_annotation_tier(tier):
                  self.__add_token_annotation_tier(tier)
  
              else:
                  self.__add_span_tier(tier)
docs: adds a tier to the document graph (either as additional attributes
          to the token nodes or as a span node with outgoing edges to the token
          nodes it represents)