Implement Event based Seeding and Multi-Threading

Pursuing the goal of running Allpix-Squared simulation’s events -independent by nature- in parallel, have led to the identification of performance bottlenecks that prevent Allpix-Squared from fully utilizing available CPU cores and from scaling the execution time relative to the number of used cores. In this year GSoC, I propose to continue working on solving these bottlenecks, most importantly the Geant4 dependency by implementing a custom run manager encapsulated within Allpix-Squared that would fix the scalability issues and allow for running events in parallel.