ELO rating

[Jameskmonger]

Players and bots should have an ELO score

This should be calculated by getting their ELO difference to each other player based on the difference in their finishing positions, then averaging this to change the players ELO

[Jameskmonger]

https://sradack.blogspot.com/2008/06/elo-rating-system-multiple-players.html

https://gamedev.stackexchange.com/questions/55441/player-ranking-using-elo-with-more-than-two-players

[Jameskmonger]

• new lerna package "@creature-chess/elo"
• take in an object like:

	{
		"player-id-abc123": { ranking: number, position: number },
		"player-id-iej917": { ranking: number, position: number },
		/* ... */
	}

• return an object like:

	{
		"player-id-abc123": { ranking: number },
		"player-id-iej917": { ranking: number },
		/* ... */
	}

• break the maths up into separate functions and unit test them (simple maths calculations should be very easy to test)