add a new function TKDTreeBinning::SortOneDimBinEdges and fix 
  SortBinsByDensity for the 1D caase
 This fixes ROOT-4889 and  ROOT-4890


git-svn-id: https://root.cern.ch/svn/root/trunk@49286 27541ba8-7e3a-0410-8455-c3a389f83636