Check that user specified edges have a node which exists.

def chk_edges_nodes(edges, nodes, name):
        """Check that user specified edges have a node which exists."""
        edge_nodes = set(e for es in edges for e in es)
        missing_nodes = edge_nodes.difference(nodes)
        assert not missing_nodes, "MISSING: {GOs}\n{NM} EDGES MISSING {N} NODES (OF {T})".format(
            NM=name, N=len(missing_nodes), T=len(edge_nodes), GOs=missing_nodes)