Adding `InfoLogger` to `messaging` module.

Moving this class here from `dmc.py` script, so that all scripts can
import it.
Adding `argparse_utils` module.

Moving `argparse` argument type functions here from `dmc.py` script, so
that all scripts can import them.
Adding `dmc_saturation_plot.py` script.

This script is a still a work-in-progress.