Improved DDTeleporter

Made various changes to clarify code in DDTeleporter. For instance, we
had a whole switch block that was used to give the same outcome on every
case except the default. I rewrote the code there to remove the block.
Also changed DDTeleporter.checkDestination() since it was redoing the
destination orientation checks unnecessarily, changing the entity's yaw
when it shouldn't have side effects, and some other little things.