Provide a table in the report to show the effect of repeating the event

Instead of exposing a "repeat this event" option in the calculator form (as per #89 (closed)), I simply always produce a table to show the risk of repeating this event.

Live demo at http://test-cara.web.cern.ch/calculator/baseline-model/result.

In order to achieve this, I added a repeats attribute to the newly created ExposureModel (from !74 (merged)). In effect, I multiply the total exposed quanta, and the probability and R0 values just fall out.