Add Bass Diffusion Model

[wd60622]

Add implementation of Bass diffusion model

Functionality would include:

• [ ] Model class with sampling and prediction methods
• [ ] Useful artifacts like the imitators and innovators, peak time, and inflection points
• [ ] Some plotting functionality when applicable
• [ ] custom priors and likelihoods via Prior class
• [ ] Save & load functionality via ModelBuilder mixin

Some items I am concerned about:

• The magnitude of the m parameter. Seems to fairly large

Some steps to explore after the initial implementation:

• [ ] Generalized Bass model / Adding covariates

[drbenvincent]

Because there are a variety of models which try to achieve roughly the same thing, if this is going in a new module, then perhaps it's name can be related to "product lifecycle" modelling/analysis?