levelToIndex = (level) => (
			(level < 10)  ? 0 :
			(level < 30)  ? 1 :
			(level < 60)  ? 2 :
			(level < 100) ? 3 :
			4
		)
Look up for Katori Class (and other CTs) Exp Bonus from current fleet.
@return {exp bonus modifiers}, default is 1.0
@see http://wikiwiki.jp/kancolle/?%B1%E9%BD%AC#v657609b
@see https://docs.google.com/document/d/1iiQpAyVQvnhVG-j-zx-Am41RPiZRISaL6FdHTKhYZaU/view