Produces a gaussian distributed series of number.doubles.
The subject is the mean, i.e. the average number.
The standard-deviation specifies how different the result is likely to
be from the mean.
If you have a mean of 100 and a standard deviation of 10, then
The average of the produced numbers will be 100.
68% of the produced numbers will be between 90 and 110.
95% of the produced numbers will be between 80 and 120.
99% of the produced numbers will be between 70 and 130.
100.<random_normal 10/>
will return a number.double between 90 and 110 about 68% of the time.