exploring comparison to ODE models

[eqmooring]

Making this as a draft PR just as a way to share and explain code...I'm not sure we'd ever want to actually merge this branch in, at least not in its current form.

There are three scripts here:

• create_simple_homog_pop.R, which is a very simple script I created because I ran into an issue getting create_synthetic_population.R to run (and this simpler approach makes the output from Ixa simpler)
• run_ixa_repeatedly.R, which is a little script that calls the Ixa model from R and compiles the results into a single .csv file
• make_ode_and_compare.R, which is the actually interesting script that runs ODE models and plots results; it includes a function that tries to simulate what we expect duration of infectiousness should be from the Ixa model

Here are what the resulting plots look like: