This method is called per vertex and can iterate over all of its neighboring edges
with the specified direction.
If called with EdgeDirection.OUT the group will contain
the out-edges of the grouping vertex.
If called with EdgeDirection.IN the group will contain
the in-edges of the grouping vertex.
If called with EdgeDirection.ALL the group will contain
all edges of the grouping vertex.
The method can emit any number of output elements, including none.