Enabling generation of all 4l final states in Powheg gg4l
This MR introduces a new algorithm, gg4l_emu2all, that is scheduled as a post-processing of LHE files when generating gg->4l final states using the Powheg gg4l process. This is an extension of previous algorithms that were developed to allow the generation of 4e or 4mu final states starting from 2e2mu final states, done in 2023 - see !61671 (merged).
The output of the code has been validated generating 50 events: after the Powheg generation, the gg4l_emu2all algorithm is run and the final states modified according to the injected probabilities. The LHE file and the log of the generation are attached to the MR.