Add distributions to the probability module

[kurin]

Problem you'd like to fix

The probability module has a handful of distributions. I'd like to add more. Would it be okay if I just submitted PRs adding distributions, or is there anything I should try to avoid?

Screenshots

Describe the solution you'd like

I'd probably start out with the beta and log-normal distributions, but this is really more just to see if such PRs would be welcomed or if they'd conflict with something I don't know about. Thanks!

[kurin]

Or maybe like the gamma distribution?