Add an exponential filter to obtain a flat decay time sample
In this MR, I modified the xAODDecayTimeFilter to be able to filter on long-lived particles based on their decay time.
It is done for all LLPs with the matched pdgID whose decay time is within the given range.
Two additional parameters are added:
- A flag to turn on or off the flat life time filtering
- A variable to tell what the seeding lifetime is
Random number generation is inspired by the existing functions in other filters.