Parallelize Kassiopeia particle tracking

[richeldichel]

In the last Kassiopeia user call on January 13th 2025, the topic of multithreading in Kassiopeia came up. Currently, Kassiopeia processes can only run sequentially, meaning that parallelization needs to be done outside of Kassiopeia by starting multiple instances and combining the output afterwards.

Currently, events are put into a particle queue. A while-loop iterates until the queue is empty. See KSRoot.cxx:

while (!fEvent->ParticleQueue().empty()) {
[...]
        // execute a track
        try {
            ExecuteTrack();
        }

So, multithreading should be possible, however there have not been any deep checks on thread-safety in the code.
Any ideas and/or pull requests that try to add parallelism for Kassiopeia's particle tracking are highly appreciated!