Add PromptLeptonImprovedVeto to stdm3 for 21 2

Add a flag to save only the BDT outputs of the LeptonTaggers to the leptons.

For the PromptLeptonVeto and the PromptLeptonIso, they are obsolete taggers. To save more space will only save those BDTs' outputs by default.

Add the new tagger PromptLeptonImprovedVeto to the STDM3 derivation.

@fuhe and @rustem will be responsible for this merge request.