Add cli

[jlubken]

Very small changes to models.py. So merging something else before this is just fine.

FIXME: Add a test to validate the dataframes produced --not the csvs--.

Obviously we want to rework the defaults.py at some point to incorporate:

• the settings.cfg instead of the settting.py
• maybe move the settings.cfg parser into classmethods of defaults.py
• the bounds validation that is duplicated between the cli argument parser and the ui widgets
• possibly make the Defaults/Constants object own the ranges and help text
• note that (like the settings.py) the rates in the settings file are not the percents in the ui, so the validation help text and messages are different

And later duplication once refactoring the models with dataframes and running models from the Parameters object:

• how sim_sir and the utils are run by both the cli and the app

And if we want to output graphs:

• we do need the max_y_axis added as required to the config file, defaults, parameters
• haven't looked into the plotting to file dependencies

Closes #171

[quinn-dougherty]

amazing, thanks!