This script has been giving people trouble

most likely because I coded it with equals parts speed and badness
I don't really understand this use of `add_mutually_exclusive_group`, since it only makes sense when you add at least two arguments to the group, and you're only adding `--sample-only` did I miss something?

From https://docs.python.org/2/library/argparse.html#mutual-exclusion:
> Create a mutually exclusive group. argparse will make sure that only one of the arguments in the mutually exclusive group was present on the command line