from Gabriel: 

 - Further replacement of GetBinContent with lighter / faster RetrieveBinContent in TH1/2/3
  - Replaced 3-level loops with simple loop in TH1::Multiply where possible


git-svn-id: https://root.cern.ch/svn/root/trunk@48289 27541ba8-7e3a-0410-8455-c3a389f83636