Add beta_step into a spim_simulate_control object. Typically
this happens just before running simulations, after the Rt values
have been converted into beta values according to the assumptions
of the simulation.
spim_simulate_set_beta_step(control, beta_step)A control object from spim_simulate_control
Beta values; either a named list (if beta_step
appears in the run grid, which it probably will) or a single
set. These should be 3d arrays with dimensions corresponding to
particle, 2. region, and 3. step (so time multiplied by
steps_per_day)