Fixing memory leak when running BAT on data ntuples (Issue #61).

Made the changes discussed above except:

    https://github.com/BristolTopGroup/AnalysisSoftware/blob/master/src/Readers/NTupleEventReader.cpp#L236 should be initialised blindy

Seems to have not made any difference though, and it still works. 

Tested successfully on both data and monte carlo ntuples and the memory usage was the same as when running on monte carlo jobs previously.