ITS: add COVID-19 example

[drbenvincent]

Add an example based on my COVID-19 example here https://www.pymc.io/projects/examples/en/latest/causal_inference/excess_deaths.html

• [x] add first iteration of the example
• [ ] once #22 is finished, then run this NOT on standardised data, and provide appropriate priors. This way we can get more meaningful parameters and the predictions will be in the original data space.