Assign default values on attributes to all vertices.

def _set_default_vertex_attributes(self) -> None:
        """Assign default values on attributes to all vertices."""
        self.graph.vs["l2fc"] = 0
        self.graph.vs["padj"] = 0.5
        self.graph.vs["symbol"] = self.graph.vs["name"]
        self.graph.vs["diff_expressed"] = False
        self.graph.vs["up_regulated"] = False
        self.graph.vs["down_regulated"] = False