p(x) dx = {1 \over \sqrt{2 \pi \sigma^2}} \exp (-x^2 / 2\sigma^2) dx
for x in the range -\infty to +\infty. Use the
transformation z = \mu + x on the numbers returned by
gsl_ran_gaussian
to obtain a Gaussian distribution with mean
\mu. This function uses the Box-Mueller algorithm which requires two
calls the random number generator r.