Truth data storage

[sbfnk]

This issue may fall by the wayside if we end up storing truth data on Zoltar, but at the moment truth data are stored in files with names that have to be matched to the target variables (e.g., "Incident cases" for "inc case"; "incident hospitalizations" for "inc hosp" etc.). It would be good to come up with a more error-proof system, e.g. either have the target variable itself in the file name or give files arbitrary file names and store the target variable in a column in the csv files (which would also allow multiple target variables per file). Either solution would then require changes in covidHubUtils::load_truth.

[Bisaloo]

This FIXME is related:

https://github.com/covid19-forecast-hub-europe/covid19-forecast-hub-europe/blob/599b4a69e4e9bcb4ceef8b371f81a06e304bb74e/viz/prepare_truth_data.R#L7-L13