comp_histos.C does a chi-squared comparision between histograms
  in two root files. Need to create the subdirectory Output
  The execution is
  .x Macros/comp_histos.C("oldfile.root","newfile.root")

  Main purpose is for comparing histograms
  after changes to hcana. Histograms which are exactly the same
  have a chi-square = 0 .
  For histograms that fail the chi-square test, a list of
  names in placed in Output/hist_diff_NoMatch_output.txt
  and the histograms are plotted and the plots saved to
   Output/hist_diff_NoMatchoutput.pdf
  For histograms that pass the chi-square test, a list of
  names in placed in Output/hist_diff_Match_output.txt