Cobblemon: Nests & Dens

Bugfix / Tweaks
- Reworked Den Structure generation to reuse previous den locations (if that den has closed) to hopefully stop the den generation from getting out of control.
  - This should prevent it from generating dens outside world borders (if a limit is used).
- Added check to close a den if all players have left using a third party tool (item or command). 
- Fixed issue with Den guardian remaining after den was closed and opened
- Fixed issue with Den would generate the same template everytime it opened