This function returns a random variate from exponential power
distribution with scale parameter a and exponent b.
The distribution is,
\[p(x) dx = {1 \over 2 a \Gamma(1+1/b)} \exp(-|x/a|^b) dx\]
for \(x \geq 0\). For \(b = 1\) this reduces to the Laplace distribution.
For \(b = 2\) it has the same form as a Gaussian distribution, but with
\(a = \sqrt{2} \sigma\).