Tutorial #8: gather-sum-apply iterations

[vasia]

Explain the GAS model and how it's implemented in Gelly. We can use the same example as in #9 to make the comparison between the 2 models.

[vasia]

More or less the same comments apply here as in tutorial #7.
In both tutorials we should run the algorithm on the chosen dataset and provide some results / visualization.
We should use the same exercise of vertex-centric for implementation here too.
The model comparison already exists in the documentation, so there's no need copying it here again. We can simply link to it :)