[Internals] move to FrailtyGenerator

[lrnv]

This will allow to hide the ugly WilliamsonFromFrailty, with the same kind of interface as WilliamsonGenerator.

Moreover, the \phi function can be directly taken as the laplace transfrom of the random variable, I think that Distributions.jl has laplace transfrom of variables right ? Since laplace transfrom of positive random variables are always completely monotoonous, that would make a lot of sense !