Adding functionality for EM-scale SoftKiller in the trigger

This code enables EM-scale constituent modifiers to be run in the trigger by including a signal state changer in JetConstituentModSequence. This allows for the em-scale of the cluster to be modified, instead of adding an additional copy of the container, which shouldn't be done if shallow copies aren't possible in the trigger.

I have validated that I can get these jets in the output, and that the results look reasonable overall.

Tagging @khoo, who has helped discuss the strategy for this.