Probability of a large outbreak based on values of R and k from a multi-type branching process

Add functionality to calculate the probability of a large outbreak (i.e. a branching process that fails to go extinct) following an initial introduction into the population, based on estimated values of R and k. This calculation was done for COVID under different introduction scenarios, and for different baseline assumptions (which could be drawn from `epiparameter`): https://cmmid.github.io/visualisations/new-outbreak-probability

These functions could also be expanded to include multiple age groups and integration with `socialmixr` using [previously developed methodology for outbreak probability in multiple groups](https://github.com/epiverse-trace/superspreading/files/10067767/Emergence_model_2.pdf)