Add generative model tutorial.

[twiecki]

Also creates a docs subdir and moves rtlive-model.ipynb into it.

[michaelosthege]

@twiecki in #20 we create a notebooks subdir, but I think we should have just one. What do you think is more appropriate?

[twiecki]

Probably if I were to look for stand-alone docs in a repo the notebooks dir would be the first place.

[michaelosthege]

A few smaller suggestions:

1. get_and_process_covidtracking_data can be replaced with get_data from Move US-specific data loading into its own module #19:

covid.data.get_data(country='us', run_date=pd.Timestamp.today())

2. data-download (1.) can be moved far down to after the synthetic example
3. you can add a visualization of the model:

pmodel = model.build()
pm.model_to_graphviz()

4. ax.legend(frameon=False) looks nicer for cell 27
5. formatting problem in the line we will model it with a probability distribution from this study30119-3/pdf)

[k-sys]

Will merge now and fix @michaelosthege 's suggestions.