feat: Tweedie distribution

[bjoernkjoshanssen]

Perhaps Statlib should have the Tweedie distribution. This PR contains:

1. A proof that the Tweedie distribution PDF yields a probability measure, with mean μ and variance ϕ μ ^p.
2. A construction of the Tweedie random variable as a compound Poisson.
3. A proof that the constructed random variable in (2) has the distribution in (1). Hats off to Aristotle who spent four hours on that.