add argument to configure a incubation time

Currently, the date of infection is equal to the date of symptoms onset.

epidemic curves can be used to infer the probable date of exposure (if known incubation period), or calculate the incubation period (if data of exposure is known). Including the incubation period in the configuration could help use {simulist} for outbreak investigation of point source or extended source epidemics

https://github.com/epiverse-trace/simulist/blob/c9e56966a8993066e55c08c06638903a9716d9cf/R/sim_internal.R#L59

Start at R/sim_internal.R#L59, where the issue identifies the current relationship between infection and symptom-onset dates. Determine how configuration is passed into the simulation and how incubation time should affect those dates; done means users can configure an incubation period and use the resulting line list for point-source or extended-source outbreak investigation.

Written by the indexing model from the issue text.