Updating `seq-file-slice.py`

Adding a "not implemented" message and exit.
Working on `summarize_distances` function.
Moving `mkdr` function to `functions` module.

Moving `mkdr` function from `utils.__init__` to `functions` module.
Updating imports of the function in `seqdigest.py` and
`extended_test_case.py`.
Adding `argparse_utils.py` module.