Fix ascertainment vignette

[pratikunterwegs]

This PR:

1. Corrects the ascertainment vignette by re-adding filtering for early stages of the outbreak, and fixes Odd results in ascertainment plots in estimate_ascertainment.Rmd vignette #73,
2. Fixes excessive data output by showing the last few rows,
3. Fixes activate alert boxes in static_severity vignette #70 by correcting heading levels,
4. Cleans up plot style in all figures.

[pratikunterwegs]

Is this more like it @adamkucharski? - Looks familiar from older vignettes.

[adamkucharski]

Have one comment about omitting countries without weekly data, but otherwise looks good.

[adamkucharski]

This looks more plausible for most countries, but looking at the remaining 100% estimates (i.e. Spain, Germany, France, Ukraine), it's happening because the data for those four countries are only weekly, but the underascertainment algorithm (and underlying CFR calculation) is designed for daily data, which is why it's returning an erroneous result. I'd therefore suggest we omit these countries with a brief note to user about why?

[pratikunterwegs]

Oh, so that's what's going on - I thought it might be that they actually have really good reporting. I'll exclude these countries from the vignettes, but are they perhaps worth keeping in the dataset? Is there a way to modify the current methods to deal with low-frequency reporting?

[pratikunterwegs]

Converted to issue - happy to discuss further!