This solves a timing issue when browsing files containing thousands of keys

Fix for ROOT-8267 - ROOT 6 master fails to compile if clang present in system include paths, picks up system's clang includes!

the global file is not static now.

this is to implement the new design where the file is optional.

and a function DataLoaderCopy needed for Variable Importance, CrossValidation and new developments on
variable transformation

and moved to VariableTransform.h/.cxx

- removed static method from MethodBase (IGetEffForRoot),
- The way to find roots in MethodBase was modified editing the class TMVA::RootFinder
- Also was edited the MethodPDERS to find roots using the virtual method from MethodBase GetValueForRoot