Density, cumulative distribution function and quantiles for the
Tweedie distributions. Includes the normal, Poison, Poison-gamma and inverse-Gaussian distributions as special cases. tweedief produces a generalized linear model family object with any power
variance function and any power link. Includes the Gaussian, Poisson,
gamma and inverse-Gaussian families as special cases.
See: http://www.statsci.org/smyth/index.html