Add support for geometric and harmonic mean

[lulunac27a]

I want to calculate the geometric and harmonic mean of a list of numbers. Geometric mean is the product of numbers and finds the root of the product. The harmonic mean is the reciprocal of the average of reciprocals from a list of numbers.
For example: [3,4,5] has geometric mean of approximately 3.91 ((3*4*5)^(1/3)) or e^((ln(3)+ln(4)+ln(5))/3), and harmonic mean of approximately 3.83 (3/(1/3+1/4+1/5))

[johnkerl]

Super-easy to implement -- great idea @lulunac27a ! :)

[lulunac27a]

Formula for geometric mean in Go:

func geometricMean(nums ...float64) {
logSum := 0
count := 0
for _, num := range nums {
logSum += math.Log(num)
count++
}
return math.Exp(logSum / count)
}

Formula for harmonic mean in Go:

func harmonicMean(nums ...float64) {
reciprocalSum := 0
count := 0
for _, num := range nums {
reciprocalSum += 1/num
count++
}
return count / reciprocalSum
}