This function computes the probability density
of the Wakeby distribution given parameters ($\xi$, $\alpha$, $\beta$, $\gamma$, and $\delta$) of the distribution computed by pargev
. The probability density function of the distribution is$$f(x) = [\alpha(1-F)^{\beta - 1} + \gamma(1-F)^{-\delta - 1}]^{-1}\mbox{,}$$
where $f(x)$ is the probability density for quantile $x$,$\xi$ is a location parameter, $\alpha$ and $\beta$
are scale parameters, and $\gamma$, and $\delta$ are
shape parameters. The five returned parameters from parwak
in order
are $\xi$, $\alpha$, $\beta$, $\gamma$, and $\delta$.