Make TMVA thread-safe with respect to use of Reader

It is now possible to create independent TMVA::Readers and use
them simultaneously on different threads.
Training of MVAs is still only safe single-threaded. In addition,
it is not safe to use multiple instances of MethodCFMlpANN either
single or multi-threaded because of a global 'this' pointer.
Merge pull request #12 from Dr15Jones/makeTMVAThreadSafeForReader

Make TMVA thread-safe with respect to use of Reader