This is an auxiliary function for controlling arguments for the incidence rate, used
internally to specify details in s_incidence_rate().
Arguments
- conf_level
 (
proportion)
confidence level of the interval.- conf_type
 (
string)normal(default),normal_log,exact, orbyarfor confidence interval type.- time_unit_input
 (
string)day,month, oryear(default) indicating time unit for data input.- time_unit_output
 (
numeric)
time unit for desired output (in person-years).