add method to prune histoSys

While there are already several options to prune overallSys implemented in TSSystematicsManager::processOverallSys, the histoSys equivalent TSSystematicsManager::processHistoSys is still empty. A first pruning procedure is the following:

• create relative shape variation histogram as Up/Nominal and Down/Nominal. Since uncertainties are usually highly correlated between Up/Down/Nominal only the uncertainty of the variation should be considered for the ratio
• fit a 0th order polynomial to the resulting ratio ("flat hypothesis")
• prune based on the (left sided) p-Value of the Chi2 fit based on a configurable threshold (which should typically be rather low as taking only the stat. uncertainty of the variation histogram largely overestimates the uncertainty of the ratio)