Improvements to Treasurehunt landmarks

- Add Graveyard (with random graves) and Forge (working/abandoned/ruined) landmarks
- Correct issues with Cottage landmark hidden chest location
- More sensible Landmark wieghting defaults in config file
- Add willager to working forge and occupied cottage
- add check on thunts in mcdungeon.py to prevent potential rescan when you have 0 dungeons but >0 treasure hunts
- add sanity check in Landmark::clearing to avoid disc-of-air issue and disc-in-water issue. Maybe.