[Rivet] CMS rivet analyses

[Hendrik Hoeth]

FYI (found in CMS_2011_S8978280):

   // This works as long as the KShort, Lambda, and Cascade are not
   // decayed in the generator.
   const UnstableFinalState& parts = applyProjection<UnstableFinalState>(event, "UFS");

This works even if KShort, Lambda and Xi decay in the generator.
That's why it's called *Unstable*FinalState.

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