These functions provide information about the inverse Gaussian
distribution with mean equal to m and dispersion equal to
s: log hazard.
(See `rmutil` for the d/p/q/r boxcox functions density,
cumulative distribution, quantiles, and random generation).
The inverse Gaussian distribution has density
$$
f(y) =
\frac{1}{\sqrt{2\pi\sigma y^3}} e^{-(y-\mu)^2/(2 y \sigma m^2)}$$
where \(\mu\) is the mean of the distribution and
\(\sigma\) is the dispersion.