Bugfix in recursive RooAddPdf splitting (needed for Hgg), and vectorizing multipdf (off by default)

In some cases the recursive splitting of RooAddPdf with N-1 coefficients was resulting in bogus coefficients because the default constructor of RooAbsReal was used instead of the one with names (commit e7efb1cf00e9673bf38435d9daf84a8ce96ec13c).
Checked that after this fix the model gives identical results as the master branch for untagged categories in 8 TeV data (now running the full card). The previous version had been tested only with an older version of the H->gg datacard that had the RooAddPdfs always with N coefficients, and so the validation was successfull despite this bug.

In addition, this introduce all the needed things to be able to vectorize RooMultiPdf, with optimized versions of RooExponential and RooPower. The feature is not on by default, but can be switched on with
     --X-rtd ADDNLL_MULTINLL=1
Also this yields identical results (within precision) as the nominal code, and is significantly faster.
It will be switched on by default in a later commit, after further testing

@nucleosynthesis @bendavid @matthewkenzie you might want to try this out (I'll now merge also Josh's #79 into it). note that for best performance you should also create the binary workspace in the development branch, and you should pass the --for-fits option in the text2workspace if you're using it for MultiDimFit or MaxLikelihoodFit