Even if this class looks very simple now, we're trying a few things
related to these stats. Having a class for it makes future changes
easier and, if there weren't any future changes, at least it makes
current experiments easier.
Note we keep the method `participants_by_geozone` to return a hash
because we're caching the stats and storing GeozoneStats objects would
need a lot more memory and we would get an error.
