getNextRandomResult(): implement strataIds and factorLevels

[fpahlke]

In getNextRandomResult() the following features need to be implemented:

# get previous random system state
randomSystemState <- NULL 
lastSubject <- randomDataBase$getLastSubject(randomProject)
if (!is.null(lastSubject)) {
    if (is.null(lastSubject$randomResult)) {
        stop("'randomResult' of subject ", lastSubject$toString(), " does not exist")
    }
    randomSystemState <- lastSubject$randomResult$randomSystemState$clone()
} else {
    randomSystemState <- RandomSystemState()
    randomSystemState$init(randomConfiguration$treatmentArmIds, 
        factorIds = randomConfiguration$factorIds, 
        strataIds = NULL # TODO implement strataIds
    )
}
if (is.null(randomSystemState)) {
    stop("Failed to get random system state")
}

factorLevels <- list() # TODO implement factorLevels

randomResult <- randomMethod$randomize(factorLevels, randomSystemState, randomAllocationValue)
randomDataBase$persist(randomResult)