This method has to be called INSIDE the user redefined CreateOutputObjects

 This method has to be called INSIDE the user redefined CreateOutputObjects
method, before creating each object corresponding to the output containers
that are to be written to a file. This need to be done in general for the big output
objects that may not fit memory during processing.
- 'option' is the file opening option.
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NOTE !: The method call will be ignored in PROOF mode, in which case the
results have to be streamed back to the client and written just before Terminate()
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Example:

void MyAnaTask::CreateOutputObjects() {
   OpenFile(0);   // Will open the file for the object to be written at output #0
   fAOD = new TTree("AOD for D0toKPi");
   OpenFile(1);
   // now some histos that should go in the file of the second output container
   fHist1 = new TH1F("my quality check hist1",...);
   fHist2 = new TH2F("my quality check hist2",...);
}