Cannot arbitrarily set the directory for weight files

After the commit f009d8e2b9ee9ee669735bc456685b569eb7f2f4, it is no longer possible to arbitrarily set the weight file directory, as the path is forced to start from the data set name:

$ git diff f009d8e~ f009d8e tmva/tmva/src/MethodBase.cxx

...

-   SetWeightFileDir( gConfig().GetIONames().fWeightFileDir );

-   gSystem->MakeDirectory( GetWeightFileDir() );

+   fFileDir=fDataSetInfo.GetName();

+   fFileDir+="/"+gConfig().GetIONames().fWeightFileDir;

+   SetWeightFileDir(fFileDir);

+//    SetWeightFileDir( gConfig().GetIONames().fWeightFileDir );

...

This code requires the data set directory to be a first-level subdirectory of the directory from where TMVA is started, which is not necessarily the case. In order to restore this feature, one can simply avoid setting the prefix, or add a member variable containing the top-level directory name (see attached patch, likely incomplete).