Egonet a.k.a. per-edge neighbors

[leotrs]

I'm moving the conversation from #32 to here.

The current is a proof of concept. We just have to agree on the name and whether we want an analogous method for edges. Once we agree on that, I'll implement those and add tests.

Demo:

In [6]: H = xgi.Hypergraph([(0,1,2), (1,2), (0, 3)])

In [7]: H.neighbors(0)
Out[7]: {1, 2, 3}

In [8]: H.egonet(0)
Out[8]: [[1, 2], [3]]

In [9]: H.egonet(0, include_self=True)
Out[9]: [[0, 1, 2], [0, 3]]

Closes #32.

[iaciac]

Looks clean to me! :)

[maximelucas]

Looks good. Do we want H.neighbors(0) to return a set or a list (of unique values)?

[leotrs]

I think a set is more meaningful because it makes it more readily apparent that each neighbor appears only once.

[leotrs]

Alright since you both think it's good - this is ready for review. Any thoughts on the name of the method?

[maximelucas]

I think it's good. Regarding the name, I would not intuitively go looking for H.egonet() when wanting its output. But I think since we linked both functions in the doc, it's okay.

Do we want to add the order parameter in both of those? Can be later too. It already works as it is.

[leotrs]

Oh good call - that makes me think we might want to leave this for later so we may have a systematic way of filtering edges by order across the entire codebase...