New data member fFileNumber containing the current file number (default=0).
New static member Int_t fgMaxTreeSize (default value 1.9 Gbytes).
New functions ;
  Int_t  GetFileNumber()
  Int_t  GetmaxTreeSize()
  void  setMaxTreeSize(Int_t maxsize)

The automatic file overflow is called by TTree::Fill and implemented
in the new function ChangeFile:
TFile *TTree::ChangeFile(TFile *file)
{
  // called by TTree::Fill when file has reached its maximum fgMaxTreeSize.
  // Create a new file. If the original file is named "myfile.root",
  // subsequent files are named "myfile_1.root", "myfile_2.root", etc.
  //
  // Return pointer to new file
  // Currently, the automatic change of file is restricted
  // to the case where the Tree is in the top level directory.
  // The file should not contain sub-directories.
  //
  // Before switching to a new file, the Tree header is written
  // to the current file, then the current file is closed.
  //
  // To process the multiple files created by ChangeFile, one must use
  // a TChain.
  //
  // fgMaxTreeSize can be set via the static function TTree::SetMaxTreeSize.
  // The default value of fgMaxTreeSize is 1.9 Gigabytes.
  //
  // If the current file contains other objects like TH1 and TTree,
  // these objects are automatically moved to the new file.
  //
  // IMPORTANT NOTE:
  // Be careful when writing the final Tree header to the file!
  // Don't do:
  //  TFile *file = new TFile("myfile.root","recreate");
  //  TTree *T = new TTree("T","title");
  //  T->Fill(); //loop
  //  file->Write();
  //  file->Close();
  // but do the following:
  //  TFile *file = new TFile("myfile.root","recreate");
  //  TTree *T = new TTree("T","title");
  //  T->Fill(); //loop
  //  file = T->GetCurrentFile(); //to get the pointer to the current file
  //  file->Write();
  //  file->Close();


git-svn-id: http://root.cern.ch/svn/root/trunk@5143 27541ba8-7e3a-0410-8455-c3a389f83636