new batch of updates for CombineTools

 * Re-added set of filter methods without the bool argument to
   CombineHarvester
 * Added small functions to read tables of cross sections and
   branching ratios and convert these to labelled TGraph objects.
   These are then used by a function to scale the rate of signal
   processes.
 * Added an improved and cleaner system for generating systematic
   uncertainties which makes use of variadic template functions.
   Previous attempt to be removed shortly. Implementation is now
   in its own header file interface/Systematics.h
 * Complete mu-tau channel systematics for the legacy SM analysis
   provided by function with interface in interface/HttSystematics.h
   and implementation in src/HttSystematics_mt.cc.