Tidy up entity geometry stuff:

- Add a getCentre method for entities to GeometryUtils
- Replace getPositionEyes(0) with getPositionEyes(1) since it's more efficient, and by definition they must be the same
- Remove all the silly getEntityBoundingBox().minY stuff, this bug has been fixed since 1.8! (which begs the question: if it was considered a bug, how on earth did it go unfixed for that long?)
- A few other small things

src/main/java/electroblob/wizardry/spell/WitherSkull.java

@@ -74,7 +74,7 @@ public class WitherSkull extends Spell {
 				EntityWitherSkull witherskull = new EntityWitherSkull(world, caster, 1, 1, 1);
 
 				double dx = target.posX - caster.posX;
-				double dy = target.getEntityBoundingBox().minY + (double)(target.height / 2.0F)
+				double dy = target.posY + (double)(target.height / 2.0F)
 						- (caster.posY + (double)(caster.height / 2.0F));
 				double dz = target.posZ - caster.posZ;