Running DSA/PSA for a demographic analysis

[jrdnmdhl]

In some cases, it may be important to have a model which takes into account multiple groups of patients even in DSA and PSA analyses. Perhaps this could be accomplished by making sure that the run_psa and run_dsa generics are given methods for the combined_model class, though perhaps this requires a whole new approach since one may need to account for different groups having different parameter distributions for certain variables...

[pierucci]

I agree. I'm going to merge #141 into that issue.

[Labeit]

It is probably still an open issue to run run a DSA/PSA for a demographic analysis!?