Update lepton truth decoration

Adding some improvements on top !300 (merged), which also closes #70 (closed).

1. Rename MuonTruthDecorator->LeptonTruthDecorator since we are also decorating information about electrons.
2. Remove softMuon_ from variable names which are not using soft muon information.
3. Move reco and truth lepton decoration calculations outside of the if (softMuon_link.isValid()) { block so they are given sensible values even in the absence of a soft muon.
4. Decorate variables to the jet, rather than the btagging object for consistency, and to make these vars writeable.
5. Ensure that dR selection is a applied to the calculation of n_prompt_leptons and n_non_prompt_leptons (before these were counted event-wide). I also now apply the selection to dr_prompt_truth_leptons and dr_non_prompt_truth_leptons, but we can remove that if necessary.
6. Remove _in_max_dr in variable names for consistency and since the dR selection is heavily implicit.
7. Some some if statements that weren't doing anything and move variable declarations closer to where they are used.
8. [Controversial?] Replace the hardcoded dR<0.4 selection with the configured minimum_lepton_dr cut which is also used for the ttbar OR, which is usually set to 0.3 in the current configs.
9. Add nPromptLeptons to EMPFlow.json giving us a nice way to distinguish electrons as shown in #70 (closed).

Tagging @frenner and @dguest who can let me know if I have been overzealous with the changes.