fix for cross coerrelation fuction

[Sauravroy34]

fixes #817
we implement our normalisation method from
#634 (this pr introduced)
1 )https://academic.oup.com/mnras/article/470/3/3591/3747505

changed mean(err) **2 to var(err) as per the publications

so for similar light curve the max value is 1 at time lag = 0
example snippet

import numpy as np
from stingray import Lightcurve,CrossCorrelation
import matplotlib.pyplot as plt

times = np.arange(1000)
counts = np.random.poisson(100, size=len(times))

lc = Lightcurve(times, counts)

corr = CrossCorrelation(lc, lc, norm='variance')

corr.plot()
plt.show()

[Sauravroy34]

result of the code