OpeningMinCohorts opens backfill cohorts when the overall number of
cohorts treated so far in the trial reaches or exceeds a minimum threshold.
This can be used to implement a "delayed backfill cohort opening" rule.
Slots
min_cohorts(
integer)
the minimum number of cohorts that must have been treated before backfilling can be opened.
See also
Opening and the other subclasses listed in there.
