GetTrainTestSplitMat: Make random partitions
Description
Make random partitions, returns matrices indicating
whether the observation is in train/test for each run
note: try to get a balanced sampling for each group (classification)
or each quantile (regression). This is very useful for unbalanced data
Usage
GetTrainTestSplitMat(y, propTraining = 2/3, nRuns = 30)