Improvement to the handing of TH1s

 - Observation, Process and Systematic classes are now responsible for
   ensuring any TH1 passed in does not have negative bins, as well as
   performing the scaling to unit integral
 - The set_shape methods now take an extra bool which determines in the
   norm value should be updated to the histogram integral or not
 - New methods for getting clones or copies of the internal TH1 are
   added
 - basic implementation of flags to control the behaviour of
   CombineHarvester added. Currently plan to hard-code these to
   something sensible and add an interface later