Models with Poisson constraints don't have noRounding=true

When defining a model with a Poisson constraint with a global observable, the corresponding Poisson constraints is evaluated with noRounding=false, which means that the global observable value from the Asimov data set is treated as an integer, resulting in a wrong estimation of CLb and expected limits. 

This is is not the case when defining the global observable as a real observable, because in that case the Poisson term has the flag noRounding=true.

This has been reported in
https://root-forum.cern.ch/t/roostats-hypothesis-inversion-demo-on-off-problem-how-to-handle-observables/40318/

The attached macros also show the problem.

The attached macro show a