WIP: Include the incidence rate into report

as described in #46 (closed) we would like to include the probability that we have someone infected on-site.

The initial idea was to add this into models (c.f. !197 (closed) ), although @nimounet correctly suggested to add to the report instead since this is a course implementation and a more sophisticated approach shall be made before adding to the model.

I discussed with Marco and we came with this approach of frequency. So we basically scale the P(I) to the probability that an infected person comes to CERN by multiplying the incidence rate (e.g. 100 per 100 000 inhabitants, which today could be a simple UI input or hardcoded value) with the number of exposed persons.

This MR is just for the report generator. we need to see the tests and then the UI field