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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.30%. Comparing base (991e488) to head (57f8889).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #893      +/-   ##
==========================================
- Coverage   96.06%   94.30%   -1.77%     
==========================================
  Files          48       48              
  Lines        9895     9895              
==========================================
- Hits         9506     9331     -175     
- Misses        389      564     +175     

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