Add Black Death disaster.

This adds a Black Death disaster, which destroys the inhabitants housings. Can be cured with medical herbs by the doctor. 
Missing:
- [x] a test
- [ ] correct AI behaviour
- [x] ~~decrease happiness~~
- [x] no trading allowed with this settlement - free trader
- [x] no trading allowed with this settlement - handling for the AI

Optional:
- [ ] 90% infection rate for ships trading with an infected settlement, and again 90% chance to infect a settlement with an infected ship