Hypergraphs

Is there any intention to support hypergraphs? The main deadlock to integrate hypergraphs in Graphs is the assumption that edges point to 2 vertices. Currently is weird to implement a "undirected" HyperEdge type: dst and src have no point there (even in an undirected graph dst and src look weird) because there will be more than 2 vertices.

My proposal is the following:

1. See simple edges as a special case of hyperedges whose cardinality is 2.
2. Stop using dst and src for undirected edges $\rightarrow$ Use vertices(edge) where possible instead.
3. For graph algorithms that need a non-hyper graph view, a hyperedge can be seen as the powerset of cardinality 2 of the connecting vertices (i.e. all vertices in an hyperedge are at distance 1).

If we relax this assumption, open edges could also be supported and implemented as cardinality 1 hyperedges.

Start by reviewing the GraphsBase.jl interfaces that assume edges connect exactly two vertices, especially uses of dst, src, and vertices(edge). Compare the three proposed changes and determine a consistent hyperedge model; done means the design is agreed and hypergraphs, open edges, and non-hypergraph algorithm views are supported without breaking existing edge behavior.