Adds primary text string onsets/offsets to all nodes that represent
        tokens. In SaltDocuments, this data was stored in TextualRelation
        edges only.

def _add_offsets_to_token_nodes(self):
        """
        Adds primary text string onsets/offsets to all nodes that represent
        tokens. In SaltDocuments, this data was stored in TextualRelation
        edges only.
        """
        for edge_index in self._textual_relation_ids:
            token_node_index = self.edges[edge_index].source
            self.nodes[token_node_index].onset = self.edges[edge_index].onset
            self.nodes[token_node_index].offset = self.edges[edge_index].offset