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:

1. A flag to turn on or off the flat life time filtering
2. A variable to tell what the seeding lifetime is

Random number generation is inspired by the existing functions in other filters.