Add incubation period to simulation

[joshwlambert]

This PR addresses #253 by adding an incubation_period argument to the sim_linelist() and sim_outbreak() functions. These functions now allow the date/time of symptom onset to be some delay after the infection time by supplying an incubation period distribution (can be a function or an <epiparameter> object like the other delay distribution arguments in {simulist}).

This changes is backwards compatible as the default incubation period is set to 0, but could be considered a breaking change due to a new argument being added to the middle of the function signatures of sim_linelist() and sim_outbreak(), so any users that are calling these functions with arguments specified by position may encounter issues.

.sim_internal() is updated to add the incubation_period when defining the $date_onset.

.check_sim_input() is updated to check the incubation_period argument, and as_function() is updated to convert the incubation_period if it's NULL or an <epiparameter> object.