Prep CombineTools for migration to HTT limits pkg

 - CombineTools no longer depends on other packages
 - Add convenient set-generating methods to CombineHarvester
 - Consistent usage of $VARIABLE style patterns instead of {VARIABLE}
   for the SetFromBinName and ParseDatacards functions
 - Add "scale" property to the nuisance class, to handle the case
   of scaled gaussians for shape systematics
 - also adapt datacard parsing and writing, AddSyst, and
   evaluate methods to handle this
 - Added a SystMapAsymm class to handle asymmetric
   uncertainties
 - Allow $CHANNEL or $BIN for the category name when
   calling LoadShapes
 - When parsing datacards throw an exception if the
   systematic does not have type lnN, shape or shape?