Code smells in `main.py`

1. [bilby_pipe/main.py] seems to have gotten bloated. A starting fix might be to move the different classes into separate modules.

2. create_main_parser() is just calling another function with a parameter (no other work is being done). Why isn't the original function used instead?

3. Line 1097 parse_args and get_command_line_arguments both come from utils.py, but have been imported in different styles. Maybe keep consistent importing?