Add stats logger

[MaryanMorel]

Again, code already exists.

The goal here would be to produce stats, and then to choose if we want to log them as txt, or plot them. While the plots cannot really be tested, it would allow some testing on the log part.

[ysebiat]

One solution would be to see the "plotter" and the "logger" as "interpreters" of the result. Thus, we will assume that the aggregator gives a result that can be tested. What do you think about this solution?

[MaryanMorel]

Yes, that's what I had in mind!