Create validation function for distribution parameter domains

[wleoncio]

From René's e-mail:

One thing missing is a function that cheks if data are inside the domain of the given domain (e.g. Normal/Gaussian is real numbers, Gamma positive real numbers, Poisson is non-negative integers etc)

[wleoncio]

There's a commented-out draft of a validity check for the rtrunc class here. If we're to continue using S4, that's the place to implement this.