Request for LogSumExp function

Could a LogSumExp function be included please?

**Motivation**
Working with small probabilities can quickly lead to underflow problems. The LogSumExp function is often used as a way to avoid this. There is a definition of the function and motivation for its use in numerical computing on the [Wikipedia page](https://en.wikipedia.org/wiki/LogSumExp#log-sum-exp_trick_for_log-domain_calculations).

**Proposed solution**
A suitable place for this function might be [here](https://hackage.haskell.org/package/math-functions-0.3.4.1/docs/Numeric-SpecFunctions.html#g:5).

Here is an example implementation

```
-- | The log-sum-exp function
logSumExp :: (Floating a, Ord a) => [a] -> a
logSumExp xs = x' + log (sum [exp (x - x') | x <- xs])
where x' = maximum xs
```