Access toy datasets generated by RooStat::FrequentistCalculator

[lukash]

@moneta

For the other questions, I think in case of zero events no fit will be performed and probably a default value will be returned for the test statistics, but this needs to be checked. RooStats will not do anything peculiar, will create an empty data set and then depends on RooFit and the test statistics used on the handling of this empty data set

I did a simple test and set the number of background events in the actual data to only two events. Plotting the value of the profile likelihood test statistics vs the number of generated events (value of the NumEventsTestStat) for the alternative hypothesis (background only) clearly shows that toy datasets with 0 events are set to a likelihood ratio of 0.