This function computes the cumulative probability or nonexceedance probability
of the Slash distribution given parameters ($\xi$ and $\alpha$) of the
distribution provided by parsla or vec2par. The cumulative distribution function of the distribution is$$F(x) = \Phi(y) - [\phi(0) - \phi(y)]/y \mbox{,}$$
for $y \ne 0$ and
$$F(x) = 1/2 \mbox{,}$$
for $y = 0$ and where $f(x)$ is the probability density for quantile $x$, $y = (x - \xi)/\alpha$, $\xi$ is a location parameter, and $\alpha$ is a scale parameter. The function $\Phi(y)$ is the cumulative distribution function of the standard normal distribution function, and $\phi(y)$ is the probability density function of the standard normal distribution.