Made the aggregation operation more general.

It used to crash when a vertex wasn’t inspect able so far.
The console is still wonky for non-inspectable vertices, but we’re
closer to having it work now.
Added a memory efficient data flow vertex.
Moved the mem. eff. splay int set to its own file.

Also improved the parameters based on TripleRush benchmarking.
Using the Kryo serialiser for console snapshots.
Added a new efficient PageRank implementation.

Uses splay int sets. Removed the old graph splitter, inserts are pretty
fast now, while maintaining the memory efficiency.
Removed dupl. MemoryEfficientSplayIntSet class.