Sampling methods example

This is the first commit for a basic implementation of a generic rejection sampler, and a more specific example of how a rejection sampler might be extended for a specific use case.

I'm not sure if this is too specific at this point, but I tried to keep it pretty general -- rejection sampling is pretty simple, but the way you'd implement a rejection sampler is similar to how you'd implement other sampling methods.

Apologies for being behind schedule on this!