Change in pick_bins.py: scale to lumi

As discussed in the meeting, now scaling the histograms to luminosity. Results for MET with a minimum of 100 events in each bin (up to discussion):
```
The best binning for MET is:
bin edges = [0.0, 27.0, 52.0, 87.0, 129.0, 171.0, 300.0]
N_bins    = 6
------------------------------------------------------------------------------------------------------------------------
The corresponding purities and stabilities are:
CoM = 7 channel = electron
p_i = [0.65, 0.592, 0.542, 0.545, 0.532, 0.714]
p_i (old) = [0.717, 0.589, 0.546, 0.573, 0.571, 0.702]
s_i = [0.539, 0.542, 0.638, 0.663, 0.629, 0.83]
s_i (old) = [0.595, 0.567, 0.659, 0.687, 0.67, 0.831]
N   = [1592, 2567, 1994, 771, 224, 148]
************************************************************************************************************************
CoM = 7 channel = muon
p_i = [0.648, 0.591, 0.557, 0.551, 0.538, 0.713]
p_i (old) = [0.716, 0.593, 0.554, 0.566, 0.565, 0.693]
s_i = [0.541, 0.541, 0.641, 0.671, 0.633, 0.844]
s_i (old) = [0.599, 0.566, 0.659, 0.68, 0.663, 0.828]
N   = [1675, 2726, 2292, 886, 249, 164]
************************************************************************************************************************
CoM = 8 channel = electron
p_i = [0.633, 0.556, 0.505, 0.513, 0.505, 0.701]
p_i (old) = [0.707, 0.558, 0.508, 0.541, 0.548, 0.699]
s_i = [0.504, 0.508, 0.604, 0.637, 0.601, 0.838]
s_i (old) = [0.569, 0.534, 0.63, 0.654, 0.661, 0.859]
N   = [7009, 10830, 8672, 3593, 1071, 788]
************************************************************************************************************************
CoM = 8 channel = muon
p_i = [0.636, 0.562, 0.526, 0.503, 0.507, 0.702]
p_i (old) = [0.702, 0.565, 0.522, 0.521, 0.542, 0.697]
s_i = [0.511, 0.514, 0.609, 0.642, 0.603, 0.844]
s_i (old) = [0.568, 0.54, 0.625, 0.658, 0.657, 0.829]
N   = [8073, 12741, 10886, 4213, 1254, 943]
************************************************************************************************************************
========================================================================================================================

```