Adding keep children strategy
The idea of this strategy is as a replacement for !27797 (closed)
After chatting with @jchapman , we realized that we could set up a truth strategy that only modifies the keep children setting of the truth incident. This does not modify the decision about the strategy (keep or throw away the vertex); it only modifies the setting of keep children. This can be used on top of existing truth strategies to change their behavior, for example by keeping all the decay products of an LLP or keeping all the decay products of a hadronic interaction, even in a mode where they would normally be dropped. Two example configurations are given.