Function reference
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          outbreak_model()
- Run PEP branching process model
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          outbreak_setup()
- Set up initial cases for branching process
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          outbreak_step()
- Move branching process forward by one generation
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          parameter_sweep()
- Sweep across parameters
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          scenario_sim()
- Run a specified number of simulations with identical parameters.
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          summarise_results()
- Summarise results from parameter sweep