[HF] Replace Gaus by Poisson for statistical MC uncertainties.

[ROOT-8033] When not choosing anything, constraints for statistical
errors of Monte Carlo simulations defaulted to Gaussian. Now, they will
default to Poisson, as promised in the HistFactory documentation.
https://cdsweb.cern.ch/record/1456844/files/CERN-OPEN-2012-016.pdf