[TMVA] Fix for TMVACrossValidationRegression tutorial crash (#2460)

* [TMVA] CV - fix not able to compile with cling

* [TMVA] CV - Rename variable `ce` -> `cv`

* [TMVA] CV - fix out of range crash

Method signature used for `DataLoader::PrepareTrainingAndTestSet`
triggered the generation of classes "Signal" and "Background". Since the
task was regression this caused confusion further down the processing
pipeline.

Code changed to use global selection cut instead of implicit
signal/background cuts.

* [TMVA] CV - improve documentation

* [TMVA] improve documentation of TransformationHandler::AddStats