extract_score: Find the predicted Bradley-Terry score of a player
In ss15859/tennis.wta: Predicts Tennis Matches

Finds the predicted score of a player following the use of the EM algorithm. If no such score exists for a player -1 is returned as the score for that player.

1	extract_score(player,lambda_hat,players)

`player`	id of the player for whom we would like the estimated Bradley-Terry score.
`lambda_hat`	vector contain a list of all the predicted scores from the EM algorithm.
`players`	vector containing a list of players for which there is a predicted Bradley-Terry score.