Add algorithm to split composite particle into subcombinations
This adds an algorithm that splits a particle into 2-body combinations of its basicParticle daughters, and relates those to the mother such that they can be analysed on a candidate-per-candidate basis.
Probably we can make it a bit more generic, and not cut on the basicParticle() property, but instead go for some given predicate. Probably out of the scope for this initial MR as well...
Added testing at DaVinci!988 (merged)