Allow different K values for different players

[djcunningham0]

It's common in real-world applications to use different K values for different players, e.g., based on their ratings or number of games played. For example, in chess ratings.

The multielo packages currently doesn't allow for using different K values within the same game. We could do something like add a k_vector parameter to the MultiElo.get_new_ratings function.

Adding this functionality to the Tracker object will be more difficult. We could maybe add a parameter to the process_data method that takes a function that determines the K value based on the list of Players. The user could specify which K to use for each player based on the number of players in the game, ratings for the players, experience of the players, etc.