WIP: Include the incidence rate into CARA model

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

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 model. we need to see the tests and then the UI