Adds a list of term ids a new span in the references
        @type term_span: list
        @param term_span: list of term ids

def add_span(self,term_span):
        """
        Adds a list of term ids a new span in the references
        @type term_span: list
        @param term_span: list of term ids
        """
        new_span = Cspan()
        new_span.create_from_ids(term_span)
        self.node.append(new_span.get_node())