Introduce an optimisation in the algorithm of TTreeFilePrefetch

void TTreeFilePrefetch::SetThreshold(Double_t t)
   // Static function to set fgThreshold
   // A new basket will not be registered in the cache if
   // the current (Tree entry < fEntryMin) > 100*fgThreshold
   // so t is more or less the percentage of baskets read to be considered in the cache
   // This algorithm eliminates branches where only a few baskets are processed


git-svn-id: http://root.cern.ch/svn/root/trunk@15396 27541ba8-7e3a-0410-8455-c3a389f83636