Add support for population selection based on predictive/posterior probabilities in getSimulationEnrichmentMeans/Rates/Survival

[xinzhn]

I am wondering if it would be possible to add an additional option (e.g, userDefined) for the argument effectMeasure in getSimulationEnrichmentMeans/Rates/Survival, to allow a user defined function based on effect measures other than effectEstimate and testStatistic for selectPopulationsFunction. Examples for such alternatives are population-specific predictive probabilities and subgroup-specific posterior probabilities[^1]. This might be implemented by providing additional arguments for the customized function selectPopulationsFunction to facilitate the calculation for both type of probabilities.

Just a thought for consideration: it may be convenient to borrow some arguments from calcSubjectsFunction (customized function for sample size re-calculation), but to provide for populations as well as subgroups. To be specific, we may need

• thetaH1, overallEffects, and stDevH1 for continuous endpoints.
• piTreatmentH1, piControlH1, overallRatesTreatment, and overallRatesControl for binary endpoints.
• thetaH1 and overallEffects for survival endpoints.

[^1]: Brannath, W., Zuber, E., Branson, M., Bretz, F., Gallo, P., Posch, M., and Racine-Poon, A. (2009), “Confirmatory adaptive designs with Bayesian decision tools for a targeted therapy in oncology,” Statistics in Medicine, 28, 1445–1463.

[fpahlke]

Relocated to issue [#32] since it didn't include any code changes, making the issues section more appropriate.