Poisson: split Knuth/Rejection methods

[dhardy]

Summary

Use an enum over Poisson implementations

Motivation

1484 uses the Poisson method internally, but only for small expected values.

@benjamin-lieser please merge this PR into yours and use poisson::KnuthMethod instead of Poisson.

[MichaelOwenDyer]

changes.patch Here's a patch file of my suggestions. Edit: Also added a constructor for RejectionMethod

[MichaelOwenDyer]

Great, looks good to me now haha 😄 Unless you want to also add the getter for lamdba I'd say this is ready to go!