Add AccEff helper script
We should add a helper script for the calculation of acc x eff for fiducial cross section measurements. A good starting point is attached (credits to Chengyang). Note that the detector-level preselection and the fiducial selection at detector-level needs to be disabled as we are fully based on particle-level quantities for the calculation of acc x eff. (Although we could also use the script to calculate acc x eff in general, not just for particle-level fiducial selection)