Coerce all edges to sets

[leotrs]

This will be done in order to forbid the appearance of repeated nodes in an edge (such an edge is called "loopy" hyperedge). @nwlandry and I discussed this and realized that XGI incurs in non-trivial performance costs due to supporting loopy hypergedges, however the number of applications where loopy hyperedges is necessary or useful is very few. So the tradeoff is just not there. We have decided to NOT support loopy hyperedges from now on. In essence, this means that the members of each edge (and also the memberships of each node) will be stored as sets. So even if the user passes in a list or some other collection, it will be coerced into a set.