fix unstable confidence intervals in {EpiNow2} output

[avallecam]

output available at https://epiverse-trace.github.io/tutorials-middle/delays-functions.html#use-an-incubation-period-for-covid-19-to-estimate-rt

sample figure output pasted in reply comments

The CI on the plot fig/delays-functions-rendered-unnamed-chunk-17-1.png have changed quite dramatically. Is this expected?

b2ec9db#diff-c8ccb4af4f83edac20865ecd44127afcd1986c35efb7c98399c6684abccc0897

Originally posted by @Bisaloo in #163 (review)

[avallecam]

for the record, after merging #163 the output was generated as expected