Add test showing calculation of standard deviation

tests/all.test

@@ -370,6 +370,10 @@ null
 [4,9]
 [2,3]
 
+(add / length) as $m | map((. - $m) as $d | $d * $d) | add / length | sqrt
+[2,4,4,4,5,5,7,9]
+2
+
 def f(x): x | x; f([.], . + [42])
 [1,2,3]
 [[[1,2,3]]]