http://root.cern.ch/root/htmldoc/src/TMVA__MethodBDT.cxx.html
In void TMVA::MethodBDT::InitEventSample( void )
Bug: the first event with negative event weight is always rejected, independent of IgnoreEventsWithNegWeightsInTraining() (also the message that negative event weights are not used always appears and is wrong)