function mean(x) {
    if (x.length === 0) {
        throw new Error("mean requires at least one data point");
    }

    return sum(x) / x.length;
}
The mean, _also known as average_,
is the sum of all values over the number of values.
This is a [measure of central tendency](https://en.wikipedia.org/wiki/Central_tendency):
a method of finding a typical or central value of a set of numbers.

This runs in `O(n)`, linear time, with respect to the length of the array.

@param {Array<number>} x sample of one or more data points
@throws {Error} if the length of x is less than one
@returns {number} mean
@example
mean([0, 10]); // => 5