`cumulative` for `Hist2D`

[Moelf]

trafficstars

    m = bincounts(hist_minDb_fc)
    cumulative_bincounts = reverse(cumsum(reverse(m; dims=1); dims=1); dims=1)
    sm2 = sumw2(hist_minDb_fc)
    cumulative_sw2 = reverse(cumsum(reverse(sm2; dims=1); dims=1); dims=1)

    Hist2D(; bincounts=cumulative_bincounts, binedges=hist_minDb_fc.binedges, sumw2=cumulative_sw2, nentries=nentries(hist_minDb_fc))