data.tables are returned silently

[sbfnk]

E.g. running the readme followed by res will not produce any output until res is called again.

This can be fixed by returning res[] instead of res e.g. in https://github.com/epiforecasts/ringbp/blob/972bf86544e716b52d0da9f60f4681372760e9ec/R/scenario_sim.R#L77