Implement methods related to pileup mitigation (SoftKiller)

[mattleblanc]

Pileup mitigation techniques have proven to be crucial parts of the reconstruction chain at the LHC -- both ATLAS & CMS have used them regularly since Run 2:

https://arxiv.org/abs/2009.04986

https://arxiv.org/abs/2003.00503

Some of the methods, like SoftKiller (SK), are available via fastjet-contrib

https://arxiv.org/abs/1407.0408
https://phab.hepforge.org/source/fastjetsvn/browse/contrib/contribs/SoftKiller/tags/1.0.0/

It would be great to port these into this library, particularly for studies of their performance in the context of future colliders or nuclear collisions.