Add a new ThOr::Combiner<T>

The main MR of this series..
depends on:

• LHCb!2668 (merged)
• Rec!2146 (merged)

exercised by

• Moore!583 (merged)

Changes:

• ParticleCombiner (aka ShinyCombiny) now makes smarter daughter cuts, it was doing too much work before.

Adds:

• ThOr::Combiner<T> including various instantiations for variant and non-variant types.
  • This is a vectorised N-particle combiner using SOA/zip input/output types and ThOr functors
• Sel::Fitters::ParticleVertex [so far partial] clone of @wouter's ParticleVertexFitter, adapted for explicit vectorisation and possibly-variant inputs

Removes:

• ThOrParticleCombiner, which is superseded by ThOr::Combiner<T>.