Implement weighted averaging for R2

weights are based on number of samples used to compute the R2 estimate.
Becomes important when doing cross validation on individual event types
since a given cross validation fold may have many more samples than the
others, or no samples at all.
Implement group level combined average total R2
Create functions for weighted t-test
create class for cross-validation folds
Integrate weighted t-test and RerpXvalFolds class
Add group level plotting for R2 plots
Make R2 plots look nicer
Make result gui more user friendly
Fix indexing bug