add LeptonTruthType
DileptonVariables: add hasTwoPromptLeptons isMcPromptLepton
selector: add two-mc-prompt counter and selection
do not apply the fjet/bjet veto, but store the numbers in the ntuple

Suneet says we're not sure we'll apply the fjet+bjet veto; store the
values in the ntuple and apply the vetoes offline
batch templates: try to use stdbuf

Check whether this can reduce the load on the disk array by buffering
stdout+stderr before slurm