Plagues

**What needs to be done?**

SE4 had plagues that would kill off population each turn on a planet until they are cured. I'm not too keen on the model that SE4 uses for plagues though. There were 5 levels of plagues and they could be cured by an appropriate level of medical bay or medical lab facility. The medical lab should prevent the plagues but instead it just cures them after one turn. Each level of plague killed a semi-random number of people each turn with the higher level plagues killing more; not sure what the formula was for that.

But I don't like that model; I'd like something a bit more realistic. Maybe each plague level kills a certain percentage of the population each turn, or we track people who are sick vs. healthy vs. recovered. And medical bays should be able to cure higher level plagues, albeit more slowly. Maybe even not make curing of plagues instant except for say level 5 medical bays curing level 1 plagues? Maybe a medical bay can reduce the level of a plague before curing it outright? A mod setting for "turns to cure equal level plague" and then a level 5 medical bay would cure a level 1 plague 5 times as fast while a level 1 medical bay would cure a level 5 plague 5 times slower?