Alex Pearce authored May 31, 2021 and Rosen Matev committed Jan 11, 2022

The ThOr::Combiner is considerably more complex as it must juggle
various sets of indices in order to support SIMD-based vectorisation.
The NBodyParticleCombiner operates on legacy LHCb::Particle objects for
which SIMD support is not foreseen. This allows a simpler
NBodyParticleCombiner implementation while keeping the concepts in
ThOr::Combiner. A brave code spelunker can read either combiner to
understand the mechanics of the combinatorics loop and vertex fitting.

Moved from Phys@6935a8e4