A lobster is a tree that reduces to a caterpillar when pruning all
leaf nodes. A caterpillar is a tree that reduces to a path graph
when pruning all leaf nodes; setting p2 to zero produces a caterillar.
Parameters:
n (int) – The expected number of nodes in the backbone
p1 (float) – Probability of adding an edge to the backbone
p2 (float) – Probability of adding an edge one level beyond backbone
seed (int, optional) – Seed for random number generator (default=None).