Remove the outdated (and not flexible) Superspreading event from the expiration types. It is replaced by an optional parameter to fix directly the emission rate in the InfectedPopulation class, in case one needs to fix directly a known emission rate (it is useful in tests, at least).
Superspreading events were not used except in a few tests, which were all updated accordingly.
This is ready but requires first to merge !255 (merged)
It is also needed for the coming evolution into a diameter-dependent model.