Improve example models

[artgoldberg]

• Create a tutorial example that is developed in a step-by-step tutorial fashion.
• Use multiple Python data science tools in the example.

See this feedback from a reviewer of the JOSS paper:

I do think there is room for improvement with the model examples given. They are given as complete models, with no description of the process for creating a model. There are a lot of python features that model developers can take advantage of, but it may be hard to discover these without direct support from the de_sim team.

[artgoldberg]

"Create a tutorial example that is developed in a step-by-step tutorial fashion." is done in the updated 2. An epidemic model using DE-Sim.ipynb.