This function returns all directions with a given ID. Directions that share
an ID are not necessarily related, especially if not filtering by route type.
It's advised to use to the directions_on_route function to
search for directions of interest.
Optionally filter results by a route type. A route type can
be provided either as a non-negative integer code, or as a character:
"Tram", "Train", "Bus", "Vline" or "Night Bus". Character inputs are not
case-sensitive. Use the route_types function to extract a
vector of all route types.
user_id
Integer or character. A user ID or devid provided by Public
Transport Victoria. Refer to ?ptvapi for more details.
api_key
Character. An API key, with dashes, provided by Public
Transport Victoria. Refer to ?ptvapi for more details.