Adjust command-line arguments in train_using_detecto to allow for easier testing

[KKhaghani]

Currently, command-line arguments in train_using_detecto.py are baked firmly into the main method. These should be kept in main, and the rest of the main method moved to a function that takes the command-line arguments as parameters instead.
This paves the way for unit tests in the future: calling the function with any arguments is much simpler than entering command-line arguments each time.