tokenizes every RS3 segment (i.e. an RST nucleus or satellite).
        for each token, a node is added to the graph, as well as an edge from
        the segment node to the token node. the token node IDs are also added
        to ``self.tokens``.

def __tokenize_segments(self):
        """
        tokenizes every RS3 segment (i.e. an RST nucleus or satellite).
        for each token, a node is added to the graph, as well as an edge from
        the segment node to the token node. the token node IDs are also added
        to ``self.tokens``.
        """
        for seg_node_id in self.segments:
            segment_toks = self.node[seg_node_id][self.ns+':text'].split()
            for i, tok in enumerate(segment_toks):
                tok_node_id = '{0}:{1}_{2}'.format(self.ns, seg_node_id, i)
                self.add_node(tok_node_id, layers={self.ns, self.ns+':token'},
                              attr_dict={self.ns+':token': tok, 'label': tok})
                self.tokens.append(tok_node_id)
                self.add_edge(seg_node_id, tok_node_id,
                              layers={'rst', 'rst:token'},
                              edge_type=EdgeTypes.spanning_relation)