New ticking system & Remove command

[JustAHuman-xD]

Changes

• Added AlienRemoveCommand.java & added it as a sub command, this command will allow an operator to remove any aliens based on uuid (tabcompleted), or simply all of the aliens by putting "all"
• Overhauled the Alien Ticking system (apart from unique tick)
  • Added uuids.yml, a file for saving any alien uuids on shutdown, and loading them on startup
  • Added a new private static set called "alienIds", this contains uuids, all of the uuids of any existing entities that represent aliens
  • Instead of looping through every world and every living entity in that world it will now just loop through the alienIds set, call Bukkit#getEntity with that uuid, check if its a living entity and then check if its an alien (which should always be true unless operators manually mess with things)
  • added a couple new methods to AlienManager.java
    • AlienManager#alienIds, which returns an unmodifiable version of alienIds
    • AlienManager#addAlien, which will take one parameter, a UUID, and add it to the alienIds map, this method also tests that the uuid is of an alien entity
  • in Alien.java, in the onSpawn method it now calls addAlien with the spawned mobs uuid

Checklist

• [x] I have fully tested the proposed changes and promise that they will not break world generation, mob spawning, and the like.
• [x] I followed the existing code standards and didn't mess up the formatting.
• [x] I did my best to add documentation to any public classes or methods I added.
• [x] I have added Nonnull and Nullable annotations to my methods to indicate their behaviour for null values

[JustAHuman-xD]

Only thing is that for any servers updating to this version they have entities that are not going to be recognized right off of the bat, I was thinking about making a search command that would basically do the old implementation except it would cache any it found instead of ticking them. But not sure how well that would work.

[Seggan]

I don't think there are many aliens that people tend to keep around their house