Fix btag SF from JSON; produce latex yields tables by default (plotIt)

There was a bug (thanks @fbury and @kjaffel for reporting and investigating) - BinnedValues returns (nominal, uperror, downerror), so the scalefactor variations should be (nominal+uperror) and (nominal-downerror), this was missing for b-tagging scalefactors (but not for lepton scalefactors).

Small addition requested by @fbury : pass -y to plotIt by default (should fail silently if it can't be made).