summedHisto: bugfix (double counting first histo)
compute_fake_el_scale_factor: cache histos to a separate file

without this, the histograms are not really cached b/c the
file to store the output is being recreated after the loop
to fill the histos
add stat error band on mc
bugfix compute_fake_el_scale_factor (different names depending on mode)

mode was not being included in the cache name nor in the histoname;
this resulted in histograms and canvases being overwritten when
running without filling the histos (using hflf histos for conv).
test bjet veto
Merge pull request #58 from gerbaudo/add-err-band

Add err band