returns a single value if x is a vector, otherwise a named
vector of size = ncol(x).
Details
This function calculates the kurtosis of data which is a measure of the
"tailedness" of the probability distribution of a real-valued random
variable. Like skewness, kurtosis describes the shape of a probability
distribution. The formula used is: $$\frac{E[(X-\mu)^{4}]}{(
E[(X-\mu)^2])^{2}}$$.
This formula is the typical definition
used in many older textbooks and wikipedia