Evtpool merger

[jackal1-66]

This is a simple evtpool merger using TFileMerger (hadd engine).
The executable handles the inputs in different ways, validating the goodness of the files and merging things in batches (parallel fetching, batched merging in single pass). An example in SimRunExample is also provided.

[sawenzel]

Good starting point but maybe we should see if this is needed at all or if we could simply use ROOT's hadd.

[alibuild]

Error while checking build/O2/fullCI_slc9 for b11cc1d at 2026-08-09 11:46:

## sw/BUILD/O2Physics-latest/log
c++: fatal error: Killed signal terminated program cc1plus
ninja: build stopped: subcommand failed.

Full log here.