Consider adding `get_Zlt_pt` to TopCoffea

[bryates]

Most of our event selection methods are in the TopCoffea repo. However, since this is a bit specific, we don't have to move it.

def get_Zlt_pt(lep0, lep1, tau0):
    lt0_pt = (lep0+tau0).pt
    lt1_pt = (lep1+tau0).pt
    sfosz_l0t_mask = ((lep0.pdgId/abs(lep0.pdgId)) == tau0.charge)
    zpeak_mask0 = (abs((lep0+tau0).mass - 70.0)<20.0)
    sfosz_mask0 = (sfosz_l0t_mask & zpeak_mask0)
    lt_pt = ak.where(sfosz_mask0, lt0_pt, lt1_pt)
    return lt_pt

Originally posted by @bryates in #493 (comment)