Back in commit 383909e16, we said:
> 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.
Since there haven't been any changes in the last 5 years and we've found
cases where using the GeozoneStats class results in a slightly worse
performance, we're removing this class. The code is now a bit easier to
read, and is consistent with the way we calculate participants by age.
This reverts commit 383909e16.
bcc9fd97f5