Adding InvisiblesStrategy to JetAlg, using it rather than a bool in the useInvisibles method, and updating FastJets to use this approach for its particle filtering and to optionally use the enum as a constructor argument. The new default invisibles-using behaviour is to still exclude _prompt_ invisibles, and the default is still to exclude them all. Only one analysis (src/Analyses/STAR_2006_S6870392.cc) required updating, since it was the only one to be using the FastJets legacy seed_threshold constructor argument.