function addToMean(mean, n, newValue) {
    return mean + (newValue - mean) / (n + 1);
}
When adding a new value to a list, one does not have to necessary
recompute the mean of the list in linear time. They can instead use
this function to compute the new mean by providing the current mean,
the number of elements in the list that produced it and the new
value to add.

@since 2.5.0
@param {number} mean current mean
@param {number} n number of items in the list
@param {number} newValue the added value
@returns {number} the new mean

@example
addToMean(14, 5, 53); // => 20.5