check_time_unit: Check if the time unit is valid
Description
Will stop if not
Usage
check_time_unit(time_unit)
Arguments
- time_unit
the time unit of the branching times
"ago: "the branching times are postive,
as these are in time units ago
"since: "the branching times are negative,
as these are in time units since present
Author
Richèl J.C. Bilderbeek