Same edge (by members) gets multiple IDs in erdos_renyi_hypergraph

[maximelucas]

Working on #29 to remove singletons, I came across this:
Running

import xgi
n = 10
m = 50
p = 0.1
H = xgi.erdos_renyi_hypergraph(n, m, p)

then in H._edge, a given edge sometimes appear more than once with different IDs, e.g. { 38: [9], 39: [9], 48: [9], ...}.

In addition, not all nodes are present in edges as a singleton. For example, there is a node 8 but no singleton edge [8].

Haven't checked if #33 fixes this?

[nwlandry]

The hypergraph class currently supports self-loops and multi-edges. I would be fine with eliminating these artifacts, but as of right now, it works as I would expect, though maybe not desirable. Happy for any suggestions.

[leotrs]

I think it should be kept as is. We can always remind the user to clean up the graph after the generator returns the hypergraph object.

[nwlandry]

That sounds good to me!

[leotrs]

If @maximelucas agrees, please close this issue :)