Runs without exceptions if this object is valid.
Checks that the following properties are fulfilled:
Edges go in both directions (because the edge attributes differ).
Every edge has a ‘cw’ and ‘ccw’ attribute which corresponds to a correct planar embedding.
A node with a degree larger than 0 has a node attribute ‘first_nbr’.
Running this method verifies that the underlying Graph must be planar.
This exception is raised with a short explanation if the PlanarEmbedding is invalid.