Draft: Understand SegFaults when running selection multi-threaded

Segmentation violation occurs when running Moore with multiple threads. The new tests expose this.

Depends on LHCb!3114 (closed).

@lmeyerga tried to setup throughput tests which include selections (ThOr based) and output. However, we ran into a segfault or potentially several.

One demonstrator (Hlt2Conf.test_hlt2_thor_selections_mt) runs without writing an output file. The stack trace suggests that the segfault is in the ThreeBodyCombiner and connected to LoKi::DistanceCalculator or the geometry.

The other test activates mdf output (Hlt2Conf.test_hlt2_thor_selections_mt_output) but disables the lines using three body combiners. We also observed segfaults but the stack trace is not very enlightening.