Add Corona model using real data.

[Patrick N. Raanes]

Cross-validate with "future".

Example should say:

- DAPPER is not made to assimilate real data cases.
- It requires a truth time series for its statistics and liveplotting.
- Corona1 model is ridiculously oversimplified, using no compartments.
- It is not chaotic, but highly sensitive to the R parameter,
  which is modulated by societal intervention.
- The biggest question seems to be the modelling of the observation operator.
    - How to deal with numbers counted on different days.
    - How to deal with test volume vs number of positives.