step_simulation: Takes the simulation one dt timestep forward
In bcgov/epi.branch.sim: Simulates an Epidemic Outbreak with a Branching Process

Use the outputs of this function as input for the next step.

1	step_simulation(sim_status, state_df, rec_df, sim_params)

`sim_status`	`sim_status` list containing simulation state vector
`state_df`	`state_df` object for the simulation
`rec_df`	`record_df` object for the simulation
`sim_params`	`sim_params` object (a list) containing simulation parameters

A list containing

status - The updated sim_status object
state - The updated state_df object
record - The updated record_df object
new_sec_cases - Number of new secondary infections generated in this time step
new_imp_cases - Number of new imported infections generated in this time step