Fix simplicial contagion demo to use simplicial complex rather than hypergraph

[GitKalo]

The tutorial simplicial_contagion.ipynb generates a random hypergraph and runs an SIS contagion dynamics on it. The name is misleading, because the hypergraphs used in the tutorial are not in fact simplicial complexes. I would suggest either:

• Change the wording to "hypergraph contagion" or similar; or
• Edit the hypergraphs in the tutorial to include all lower-order hyperedges, thus making them simplicial complexes.

I could try to do either of these.