Added horizontal morphing and interpolation pds

 - basic implementation of RooFit pdf that does horizontal morphing
   between mass points automatically. Added a corresponding yield
   interpolator.
 - Added AddWorkspace, ExtractPdfs and ExtractData functions to
   CombineHarvester
 - Adapted Test.cpp to produce a single datacard with these new pdfs
   used for each signal. Temporarily disabled shape uncert. on signal
 - Added a plotter for the mass scan output