Improved enemy AI and player options

-Enemy AI now incorporate their velocity in their calculation to check when to attack (and what kind of attack they should use), so they should bite and sting with more accuracy.
-Players can now toggle what units they would like to possess with the 1,2 and 3 keys. This will have a visual indicator in the top right. The 0 key can be used to indicate players have no preference, in which case 'P' just causes players to control the nearest allied unit like before.
-Flying in and out of hives should be more robust and not cause players to fly out of the map when travelling at high speed.
-Updated larvae so that they attack more reliably (when enemies are detected in range).