from Gabriel: 

    - deprecated GetCellContent / GetCellError / SetCellContent / SetCellError -> GetBinContent / GetBinError/ SetBinContent / SetBinError provide the same functionality ; replaced these methods across ROOT (still in graf2d and main)
   - replaced SetCellError / SetCellContent / GetCellError / SetCellConten
   - refactored redundant logic (GetContourLevel, SetContent, Interpolate, SetBufferSize)
   - moved one-line methods to header file (SetMinimum, SetMaximum, GetXaxis, GetYaxis, GetZaxis, GetBinError)
   - TH2::Rebin2D has been simplified (6 for loops -> 3 for loops) => 2-3x speedup


git-svn-id: https://root.cern.ch/svn/root/trunk@48447 27541ba8-7e3a-0410-8455-c3a389f83636