change constructor of OrnsteinUhlenbeckDiffusion

[bicycle1885]

I'd like to propose changing the parametrization of the constructor of OrnsteinUhlenbeckDiffusion. The rationale behind this is that we typically want to tune the speed of convergence by changing the reversion parameter while keeping the distribution at equilibrium unchanged. Also, I think we won't tune the mean parameter in most cases because it is not very important in diffusion models. I also added a short docstring to the constructor.

[murrellb]

I also usually work with a mean,reversion,eq_std parameterization, so I'm happy with this direction. The optional arguments should allow the equilibrium std to be input directly. Maybe we call that eqσ to avoid confusing it with the volatility? So I should be able to call OrnsteinUhlenbeckDiffusion(1.0, μ = 5.0, eqσ = 10.0) where the defaults would be μ = 0, eqσ = 1 if not specified.

[bicycle1885]

I'm wondering how often we will change the parameter σ in practice. I think most users won't tune that parameter; if they do, they must know what they do. Introducing a new parameter like eqσ might be rather confusing for those who are already familiar with the volatility parameter and want to tune it for some reason.

[murrellb]

Playing with toy cases, the equilibrium std can make a practical difference for how easily the model learns. It is one of the things I tune. And anyone that knows what they're doing will understand the meaning of "eq", and can set the parameters directly if they like!