update for thesis version of mu-value likelihood scan
I've got a brand new CombineHarvester

 * split CombineHarvester member function implementation into separate
   files. _Datacards.cc for datacard parsing and writing, _Evaluate.cc
   for the GetRate, GetShape functions etc and _Filters.cc for all
   filtering type functions.
 * Re-implemented standard GetRate,GetShape,GetObservedRate,
   GetObservedShape using lookup where appropriate
 * For GetUncertainty and GetShapeWithUncertainty add overload which will
   sample sets of nuisance parameters from a fit covariance matrix, thus
   taking into account the full set of correlations
 * Supporting functions for extracting and setting sets of parameters -
   some more care needed with this behaviour under a shallow_clone()