`generateIIS()` must return an IIS, even with `setParam("iis/irreducibility", False)`

[Joao-Dionisio]

One of the assertions in test_iis.py is currently failing.

Suppose you create an IISfinder that doesn't produce an IIS (maybe your algorithm removes only some of the reasons for infeasibility). In that case, there is currently no way to not run SCIP's IISfinders. The SCIP way seems to be setting the iis/irreducibility flag to False, which calls one IISfinder and stops, even if an IIS is not found, but this is not working.

Nothing seems wrong on the SCIP side, so it's likely in the PySCIPOpt implementation.

We still decided to release, as this seemed like a strange corner case.