Add Outbreak Threshold function

The paper "Introducing the Outbreak Threshold in Epidemiology" describes a simple metric that can be used to determine outbreaks or transmission extinction. This could act as a useful function alongside `probability_epidemic()` and `probability_extinct()`.

Read the paper “Introducing the Outbreak Threshold in Epidemiology” to identify the proposed metric, then inspect the existing `probability_epidemic()` and `probability_extinct()` entry points for their conventions. Done means the outbreak-threshold calculation is available as a companion function with behavior aligned to the paper and the existing API.

Written by the indexing model from the issue text.