Replace GradoopId with Either<GradoopId, NullValue> in VertexGroupItem

[s1ck]

Summarization creates a VertexGroupItem in the first step which is further used in the algorithm implementations. This tuple type contains the vertex representative id, which is a GradoopId. When initializing the tuple, the representative id is not needed and can thus be empty. Currently, a new GradoopId is set, which needs 128 Bit per tuple and leads to more communication overhead. This can be replaced using the Either type.

[p-f]

I'll have a look at this (I'm assuming you're currently not working on it)

[timo95]

I tried it. Either is not serializable.