Parameter Description: Select Populations Function
Optionally, a function can be entered that defines the way of how populations
are selected. This function is allowed to depend on effectVector with length populations
stage, conditionalPower, conditionalCriticalValue, plannedSubjects/plannedEvents,
allocationRatioPlanned, selectedPopulations, thetaH1 (for means and survival), stDevH1 (for means),
overallEffects, and for rates additionally: piTreatmentsH1, piControlH1, overallRates, and
overallRatesControl (see examples).