from Gabriel: 

 - speedup of SetContent: using UpdateBinContent instead of SetBinContent
   - added protected method SetBinErrorSqUnchecked -> First use in TH2::Rebin2D (1.5-2x) speedup when you have errors


git-svn-id: https://root.cern.ch/svn/root/trunk@48454 27541ba8-7e3a-0410-8455-c3a389f83636