`diversity_stats` doesn't have a `max_cohort_size` parameter

[jonbrenas]

diversity_stats returns this kind of warning when a cohort is bigger than cohort_size: "Cohort downsampled from 16 to 10 samples. Set max_cohort_size=None to disable downsampling." However, it doesn't have a max_cohort_size, despite cohort_diversity_stats (which it calls) having it.

[adilraza99]

@jonbrenas diversity_stats is missing max_cohort_size (and min_cohort_size) in its parameters. It calls cohort_diversity_stats internally, but since these aren’t exposed, there’s no way to control the downsampling from the top-level function, even though the warning suggests setting max_cohort_size=None.

Other multi-cohort functions like pairwise_average_fst and h12_gwss already pass these through, so handling it the same way here would keep things consistent.