Remove CombineTools and CombinePdfs from the framework - migrate to HTT limits package

@rcl11 @adewit 
It's become tricky to keep this code in sync since parts were migrated to the HTT limits package. I think it makes sense to migrate all the remaining code in these packages and remove them from our repo. I've already copied across the latest code from this repo to the development branch, and will merge this PR tomorrow unless it's a problem for you - just let me know.

The one remaining "issue" is that I had to duplicate the new plotting routines in Core/interface/Plotting*.h into CombineTools so they get shipped to the limits package - will still want to keep these in sync by hand.