[python] mean and std

https://docs.scipy.org/doc/numpy/reference/generated/numpy.std.html

import numpy as np

arr = np.zeors(3)

arr [1] = 2

arr

>>> (0, 2, 0)

np.mean(arr)

>>> 0.66666666666666663

np.std(arr)

>>> 0.94280904158206336