Extra examples aggregations

[LucaMarconato]

"Very work-in-progress" PR where I show some extra examples with aggregation, in particular aggregating into a grid.

We could also put that code into a new notebook on rasterization vs vectorization (wip with @quentinblampey).

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[LucaMarconato]

I have simplified the aggregation notebook by using the new simpler dataset form @lopollar (thanks again!) instead of the artificial ones, that have been reported to be confusing.

Also I added a section on aggregating instances into grids, since it's a popular task.