swift234

[gardening] Improve readability of `pstdev` function

This doesn't change how the function works, just makes it easier to follow:

* Add a docstring that explains how standard deviation is computed
* Use `sample` instead of `l` for the input variable
* Use `math.sqrt` for readability
* Break it down into a couple of intermediate steps for ease of comprehension

Plus a nicer error message in the case where you try to get the standard deviation of an empty list (previously you got a not-completely obvious ZeroDivisionError).