Changes to base turfs and fixing a small bug
Created by: DrCelt
Tested 100%*
- made set-base-turf able to mass replace the old with the new (with a warning it'll cause lag, and it will - it causes about 30-45 seconds of freeze on local testing, but as it should only ever be called once unless admins are awful this is acceptable)
- made set-base-turf call a proc change_base_turf so that cool shit can come in the future (I've an idea in mind for later)
- fixed a bug where pine trees could appear on tiles with stuff on them already
*on expected differences resulting from change only