create_dir: Create directories if they don't already exist
Description
Given the names of (potential) directories, create the ones that do not
already exist.
Usage
create_dir(...)
Arguments
...
The names of the directories, specified via relative or absolute
paths. Duplicates are ignored.
Value
Invisibly, a vector with a TRUE for each time a directory was
actually created and a FALSE otherwise. This vector is named with the
paths of the directories that were passed to the function.