Compute covariance with Series, excluding missing values.
Series with which to compute the covariance.
Minimum number of observations needed to have a valid result.
Covariance between Series and other normalized by N-1
>>> s1 = pd.Series([0.90010907, 0.13484424, 0.62036035])
>>> s2 = pd.Series([0.12528585, 0.26962463, 0.51111198])