Bad visibility of plots in R0 chunk

[CarmenTamayo]

Currently the transmissibility by group plot looks very squashed and it's not possible to read the numbers on the y axis or to properly see the curves (see image below)

@Bisaloo do you think this could be fixed in the code in #144 or is it an issue of the overall template?

Thanks!

[Bisaloo]

The rmarkdown chunk producing this plot needs to specify a larger fig.height value: https://bookdown.org/yihui/rmarkdown-cookbook/figure-size.html

[CarmenTamayo]

This has been addressed in #144