added MLL_OSSF selection algo

Added MLL_OSSF algorithm that calculates the invariant mass of two OS electrons/muons and selects the events with the mass in user-defined range in event selection. Use keyword MLL_OSSF to call the algorithm in event selection,MLL_OSSF 80000 100000 selects events with at least 1 Z-like pair, whereas MLL_OSSF 80000 100000 veto vetoes them.

this is originally needed by 4tops analysis, to avoid checking out this package locally and hopefully could benefit other analysis as well, I'm moving it to athena in this MR.

Tagging @ravinab for approval.