add reminder not to wait until EpiNow2 finish running

epinow() in several sections ([this one for example](https://epiverse-trace.github.io/tutorials-middle/create-forecast.html#create-a-short-term-forecast)) takes almost 10 minutes to run.

Add a callout to notify not to wait until the calculation runs to continue with the tutorial. As soon as the Viewer panel its activated, everything is working ok

To fix in #47 