Investigate using covidData to get versioned truth data

[Bisaloo]

https://github.com/reichlab/covidData

This might be a good direction to get rid of the data-truth folder here.

[Bisaloo]

First step is to translate prepare_truth_data.py to R so we can nicely drop the package in place of the custom folder.

[Bisaloo]

I have a working version of this for viz/prepare_truth_data (I would still need to make it work with load_truth() used in reports) but I'm suddenly not sure it's the way to go. Switching to covidData introduces a hard dependency to Covid-19 and I don't see a nice way to specify this in a config file.

Instead, we should probably aim at loading the versioned truth from zoltar (and it can then work with any truth source). See this related issue in covidHubUtils: reichlab/covidHubUtils#246.

In case we need it the future, the first steps to switch to covidData are in https://github.com/epiforecasts/covid19-forecast-hub-europe/tree/covidData.

[sbfnk]

Yes, this makes sense to me.