Add ChargedLeptons enum and mode argument to ZFinder and WFinder constructors, to allow control over whether the selected charged leptons are prompt. This is mostly cosmetic/for symmetry in the case of ZFinder, since the same can be achieved by passing a PromptFinalState as the fs argument, but for WFinder it's essential since passing a prompt final state screws up the MET calculation. Both are slightly different in the treatment of the lepton dressing, although conventionally this is an area where only prompt photons are used.