Draft: Flatten 2+1 SVs to make 3-body SVs.
Flattens the structure of CompositeParticle
s constructed from 2+1 track combinations to make 3-track combinations. The resulting SV's position is given by the input 2-track SV position, and the 3-momentum is calculated from the track kinematics. Adapts @gligorov's charged kaon lines to use 3-track SVs. Also loosens the cuts on these combinations to make more general 3-body SVs.