Speeding up histogram postprocessing in doHadd #284
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Closing a rootfile with many histograms can take a long time; this can be sped up by using ROOT.gROOT.GetListOfFiles().Remove() (see https://root-forum.cern.ch/t/tfile-close-slow/24179)
This is already done within haddfast, but not when opening / closing the rootfile when doing histogram postprocessing
Since ROOT is not deleting the objects in memory when the file closes, they must be deleted manually instead
Tested and saw minor improvement in closing speed (30s to 6s) for a histogram file with 139k histograms; however analyses with more histograms could see larger improvement. If there are no such analyses, I will close this PR. :)