Feature/issue 3121 beta neg binomial rng #4451

Job	Run time
prim and rev tests	2h 56m 9s
mix/fun tests 1	2h 12m 49s
mix/fun tests 2	1h 37m 26s
fwd tests and non-fun mix tests	1h 0m 48s
	7h 47m 12s

Provide feedback