Implement stop decay polariation and three-body neutralino decay kinematics in Pythia8

For the pMSSM summary papers, the event generation of a complex mix of SUSY states and decays are done in MadGraph+Pythia8. Due to the complexity, MadSpin cannot be used to produce the correct decay kinematics of stop and neutralino decays. Instead there are implemented as a UserHooks in Pythia for the most common cases. This is targeted to release 21 as the pMSSM paper is reinterpreting Run 2 searches and therefore need to generate samples in MC21.

More details can be found at: https://indico.cern.ch/event/1315516/contributions/5533323/attachments/2697331/4681318/DecayImprovements.pdf