Descriptive Statistics Summary

Implement describe(x) returning a dict with:

• n, mean, median
• std: the sample standard deviation (ddof=1)
• skew: $\frac{1}{n}\sum\left(\frac{x - \bar x}{s_0}\right)^3$ using the population std $s_0$
• kurtosis: excess kurtosis, $\frac{1}{n}\sum\left(\frac{x - \bar x}{s_0}\right)^4 - 3$

Ignore NaN values.