(Re)-introduce XSIM output type for future MadGraph
In both Sim09 and Sim10 we need to reintroduce the support of XSIM output type in Gauss, dur to a physics requirement for ONLY a small set of samples to be produced with MadGraph.
Note that for Gauss-on-Gsino we will need to find a different long term solution.
The choice of XSIM as output type will have to trigger the addition of the options $APPCONFIGOPTS/Gauss/xsim.py
in the Gauss step.
cc: @ngrieser