Local helper to be reused.
check_input_file(filename = NULL, type = NULL, data_dir = NULL, year = NULL)
Character string of the filename of the .csv to read, if this is given, type and years determine whether there is a target to read, otherwise disk scan would be needed.
The type of file to be downloaded (e.g. 'Accidents', 'Casualties' or 'Vehicles'). Not case sensitive and searches using regular expressions ('acc' will work).
Where sets of downloaded data would be found.
Single year for which data are to be read